Vista is the biggest revision to Windows in over a decade. It will be a major change, not only for consumer and corporate Windows users, but for computer makers, software creators and many others downstream.

But what’s in Vista? How will it be sold? And what kind of computer will be needed to run it? Here’s a rough guide to the new Leviathan of the digital seas.

WHY VISTA?

Even after a major overhaul a couple of years back, Windows XP is a security nightmare. With Vista, Microsoft claims to have built in better security from the start, reducing — though not eliminating — the need to buy, learn and maintain add-on security software. The company says better security is Vista’s biggest advantage.

For instance, with the new program you’ll have to type in your administrator ID and password before installing software, to stop malicious software from installing itself silently. And Vista will have built-in parental controls.

Vista’s next big feature is built-in desktop search. Think of this as the Google Desktop search on steroids. From any screen, you’ll be able to start typing a search term and Vista will comb your hard disk for every document, photo, email, song and video that meets that criterion. It should be much faster and better than add-on search programs.

In addition, you’ll be able to save searches in “virtual folders,” which will automatically continue to collect files that meet your search specifications. So if you save a search for “Fountains of Wayne” as a virtual folder and check it a month later, it will contain every e-mail that mentioned the pop band as well as any photos you took at their concert and new songs by the band that you downloaded — even though none of these things existed when you first did the search.

The last major new feature is a rich new user interface. Called Aero, it includes a powerful new graphics system that enables such new extras as transparent windows, animation of certain screen elements (similar to the “funnel” effect Mac users are familiar with when closing a file) and the ability to see reduced, live views of all your running programs at once.

There are lots of smaller changes as well. For instance, there’s a dashboard with small programs (calendar, weather updates and stock tickers, among others) that run quickly, called Gadgets. There’s also new music and video player software; a new built-in Web browser with tabbed browsing; a new, free email program with junk-mail filtering; and a new photo-organizing program.

Many of these features are already available on the Apple Macintosh — some have been for years — but they will seem fresh to most Windows users.

HOW WILL YOU BUY IT?

Like past versions of Windows, Vista will be sold in two ways: The vast majority of people will get it by buying a new PC with Vista preloaded at the factory. That way, they’ll know the hardware and software are compatible. And a small percentage of people, either brave souls or those with PCs too new to replace, will buy Vista in a box and upgrade their computers manually.

Either way, Vista won’t be simple to purchase. That’s because it will come in at least five different flavors, compared with two versions when Windows XP launched in 2001. There will be two consumer versions of Vista, two business versions and one version that includes everything, called “Ultimate.” Also, two current special editions of Windows, the Tablet and Media Center versions, will be folded into some, but not all, of the five Vista editions.

For consumers, the biggest issue will be choosing between the Home Basic and Home Premium versions of Vista, either on new machines or in boxes. Home Premium will include the new Aero user interface along with all the security, search and other features described above. It will also have updated versions of the features currently included in the Media Center and Tablet editions of Windows XP. But Home Premium won’t run on most Windows PCs currently in the hands of consumers, and it also won’t run on new, low-end PCs. That’s because it requires hefty hardware to work right.

Most current PCs, and all the bargain-priced new ones preloaded with Vista next January, will be able to run only Home Basic, which is a stripped-down version of Vista. Microsoft insists that Home Basic will have the same security system and search features as Premium, but it won’t include the new Aero user interface and will probably lack some other features. In essence Home Basic will look and feel like a modestly improved version of Windows XP, even though Microsoft says there’ll be major improvements under the covers.

Power users, and those who want every option just in case, may go for the Ultimate version of Vista. It not only will roll up everything in the consumer and business versions, it may also have some added bells and whistles. Microsoft hasn’t announced prices yet.

WHAT HARDWARE WILL YOU NEED?

The stripped-down version of Vista, Home Basic, will run on fairly routine PCs, albeit ones with plenty of memory.

The Premium and Ultimate versions will likely require at least a midrange model or a high-end configuration.

Microsoft hasn’t officially released the recommended hardware specs. But I expect the company to recommend 512 megabytes of memory for Home Basic and a gigabyte of memory for Premium. Based on past experience, I advise doubling those amounts, to a gigabyte of memory for Basic and 2 gigabytes for Premium.

Another crucial hardware factor will be the computer’s video system. Basic Vista can run on any graphics hardware that creates a screen resolution of at least 800×600. That covers most bargain computers with graphics chips that are integrated with the machine’s motherboard and which share main memory. But Premium and Ultimate will run best on machines with a full-blown graphics card and dedicated video memory of at least 128 megabytes.

You will be able to run Home Basic on the slowest processors available, but for the better versions of Vista, you’ll need a processor running at a speed of at least 1 gigahertz. I would opt for as fast a processor as you can afford and for one with two “cores” rather than one. (A dual-core processor is essentially like having two processors on one chip.)

Vista will also support so-called 64-bit processors, which can gulp down more information than current machines. But I wouldn’t worry about that for now, unless you’re a power user. There’s very little 64-bit software available for consumers.

Vista may not be something to leap into right away. You may want to wait a while to see about defects and, especially, to see if it seems more secure, as promised.

Its visual display of files stored in a nested hierarchy of folders has worked pretty well — until recently.

In the past few years, computer hard disks have become huge, and average consumers have begun to accumulate thousands and thousands of files each year — far more than in the past. And that is making it much harder to find any particular bit of information buried in the old folder hierarchy.

Every time you plug a digital camera into a PC to transfer images, dozens or even hundreds of files can be added to your PC in one fell swoop. The same thing happens when you rip CDs or go on a photo downloading binge. Email is accumulating in staggering amounts, and just surfing the Web can add hundreds of files — silently cached copies of Web pages and images.

So the familiar file and folder system is buckling. Unless you’re the rare person who is meticulously organized, who creates a perfect system of orderly folders and recognizable file names, locating information on your own computer can be harder than finding it on the Web. There have always been search tools built into the Apple and Microsoft operating systems, but they were terrible — slow and inaccurate, covering only some kinds of data, not all. So you had to rely on separate search systems built into individual programs, such as email software.

But now a wave of new desktop search tools is becoming available, some built right into new operating systems and others available as add-ons. Big names are getting in the game — Apple, Microsoft, Google and Yahoo. I’ve been testing the leading candidates and previewing some future contenders. So here’s a rundown of the latest methods for finding all that lost or hidden information lurking on your hard disk.

Spotlight: This is the new universal, speedy search system built into Apple’s latest operating system for the Macintosh, called Tiger. Because it is an integral part of the operating system, which handles all files, Spotlight knows about all the key kinds of information stored on the computer. It can rapidly find words or phrases deep inside emails, Microsoft Office files, address books and calendars, Adobe PDF files and more. It can even probe the “metadata” — descriptive information — attached to song and picture files.

Spotlight is always available on the Mac, no matter what program you are in. You just click on a blue magnifying-glass icon at the top right corner of the screen, and a search field appears. As you type each letter of your search term, Spotlight begins generating results in a list of files that drops down almost instantly, organized by type of file. If you click on “Show All,” the list expands into a larger window where you can see more results, organized in almost any way you choose — by date, by person mentioned, by name or location on the computer.

On my Mac, I typed “Hawaii” into Spotlight and instantly got hundreds of hits. Every email mentioning the state came up, as did Word documents, Excel spreadsheets, the address entries of contacts who live there, calendar entries for my vacation in Honolulu and pictures of my trip. Spotlight even showed thumbnails of those pictures and, with one click, presented a slide show of the images. It also found the theme song from the old Hawaii Five-O television show in my music collection.

Because Spotlight is part of the operating system, it avoids one of the big flaws of add-on search systems — the need to periodically “index” new files in batches, a process that spins the hard drive continuously, sometimes for hours. Spotlight needs to do this only once, when you first install Tiger.

Longhorn: Microsoft plans to emulate Spotlight in the next version of Windows, code-named Longhorn. But it’s way behind Apple. Longhorn won’t reach consumers until the fall of 2006 at the earliest.

Based on previews of Longhorn I’ve seen, its planned built-in search system will also be fast and universal, and will avoid long indexing sessions. Microsoft also plans to allow more customization of searches, and many more instant previews of files, than Apple now provides. But these apparent advantages may evaporate.

By the time Longhorn arrives, Apple will likely have a second, even better, version of Spotlight.

Google: Until Longhorn arrives, Windows users must rely on add-on search software, and the best known is Google Desktop Search. It does the annoying indexing, but only when your machine is idle. Plus, it’s fast and fairly comprehensive, including even the contents of cached Web sites.

But the familiar Google search results page, which works so well for the Web, is very limiting for a desktop search. And the software offers only a crude way to sort the results and no way to preview content.

Yahoo: The big online service bought a desktop search engine from a company called X1 and re-branded it. This is a robust product, which indexes and searches many kinds of files and previews most of them in a built-in window. You can also take direct action on e-mails that turn up in your search — for instance, you can launch a reply right from within the search results.

The main problem with Yahoo’s offering, in addition to the fact that it isn’t buried deep in the operating system, is that it betrays its techie heritage. X1 was originally built for techies and hard-core search fanatics. Although Yahoo has cleaned it up a bit, the many choices and settings in the user interface may be daunting to mainstream users. Also, it has much more of the feel of a separate, heavy-duty program than the Apple or Google products.

MSN: The desktop search add-on from MSN prefigures what Longhorn will do, and it’s very good. Like Yahoo, it offers previews of most files right in its search results screen. And like Apple’s Spotlight, it is fast and presents a clean, simple interface that begins generating results as you type your search terms.

The main downside to MSN’s search is that in order to get it, you have to download and install a “toolbar suite” that lives in the Internet Explorer Web browser and adds a bunch of functionality that’s unrelated to search, which you might neither want nor need. Also, by default, MSN’s search product searches only your e-mail (which must be run by Microsoft products) and the My Documents folder. You have to tinker with settings to get it to search your whole computer, something all its competitors do by default. The limited search horizon cuts down on MSN’s indexing time and makes it look faster than it really is, but it will likely cause you to get only partial search results.

There are some other good search products out there from smaller companies, notably one called Copernic, by Copernic Technologies, which has a loyal following of users. Whichever you choose, once you trust desktop search, you may never again find yourself creating a subfolder.

The big advantage of Web mail is that any computer, anywhere, with a browser and an Internet connection can access it. The PC you’re using needn’t have an e-mail program like Outlook installed on it, and it doesn’t have to be configured for your e-mail account. The disadvantages: Since the type of e-mail software the Web providers use is essentially just a Web page, their services typically lack the power and speed of installed programs; they also place limits on how much e-mail you can save.

That’s why, for many years, Web-mail services have most often been the preference of light users attracted to the free email they offer. They’ve also been popular with people who use Outlook or another heavy-duty program at work, but who want an account on the side for personal e-mail. Lately, however, the major Web-mail offerings have gotten much better, to the point where they’re plausible candidates even for serious users. Here’s a quick rundown of how they compare. (A note on methodology: Although you can receive Web-based e-mail via a local program like Outlook Express, I tested the services in their most common mode — Web-based e-mail delivered through Web-based programs.)

All three of the major Web-mail providers now offer much more free storage than was common a couple years ago. Gmail leads with more than 2 gigabytes. Yahoo offers 1 gigabyte for free, and Hotmail provides 250 megabytes. All three also now have decent antispam and antivirus features, and they are a bit less susceptible than Outlook to being exploited by e-mail containing harmful computer code, particularly if you access them via a browser other than Microsoft’s Internet Explorer. That’s because malicious software writers have targeted Outlook (and IE) for years. But the three still lack some key features. For instance, none offers a full preview of e-mail content, though Gmail does show you a few words of each message. None allows you to set up multiple signatures you can attach to different outgoing messages. And the Web-based e-mail filters these services provide are fairly crude compared with those in local programs.

As for how they rank, Yahoo Mail takes the lead. It’s fast, and its gigabyte of free storage is more than enough to free most users from deleting old mail. I also like Yahoo’s autocompletion of addresses, as well as its folder and filter systems. Plus, its overall user interface is clean and clear.

Google’s Gmail is also pretty good, though its quirky design could put off some users — it’s clearly still a work in progress. Gmail has the most free storage of the Web-based providers, which is a big plus, and searching all that mail is fast and accurate. But a simple operation such as deleting an e-mail takes more steps than in Yahoo. Gmail’s biggest, most beguiling quirk is its insistence on displaying e-mail in “conversations,” groups that include all back-and-forth responses. This view can be useful, and most local e-mail programs offer it as an option. But inexplicably, Gmail refuses to let you view e-mail one message at a time. It also runs ads alongside every e-mail, based on a scan of the message’s contents.

Hotmail comes in last. It offers only a fraction of the free storage of Yahoo and Gmail, which, for my money, flatly disqualifies it as a serious contender.

Whichever program you choose, Web mail has finally arrived as a viable option. Let’s hope a good thing keeps getting better.