And then this year everything changed. Apple Computer, the maker of the Mac, switched to the same Intel-made processors for which Windows was designed. The new Intel-powered Macs, which began shipping in January, still come with Apple’s own operating system, Mac OS X, which is more modern and more secure than Windows XP. But these Macs can now easily run Windows too, and they run it as fast as standard Windows-equipped PCs.

Why does this matter? It means that you can switch from Windows to the Mac and still use the one or two Windows programs you require that have no Mac equivalent.

Among the Mac models that can perform this feat are the iMac and Mac Mini desktop computers and the MacBook and MacBook Pro laptops. Apple’s most powerful model, the PowerMac tower, is likely to be converted to the Intel chips by the end of the year, after which it, too, will be Windows-capable.

There are two options for running Windows on the Mac. They take different approaches, but in my tests, both have proved to be fast, stable and widely compatible with Windows software. One, from Apple itself, is free. The second, from a small Virginia company called Parallels, costs $79. But each carries an additional cost: You have to supply and install your own copy of Windows, which can run $200 or $300, depending on which version you choose.

The Apple system, Boot Camp, uses a technique called Dual-Boot. It splits your Mac’s hard disk into two sections, or partitions — one for the Mac operating system and all its programs and files, and one for Windows XP and all its programs and files. You can start, or “boot up,” the Mac in either operating system, but you can’t run both simultaneously. To switch between a Mac program and a Windows program, you have to reboot the computer.

The Parallels system, called Parallels Desktop for Mac, uses a technique called Virtualization. It creates a virtual Windows computer inside a window within the Mac operating system. Parallels runs Windows a tad more slowly than Apple’s Boot Camp, but allows you to run both operating systems, and compatible software, simultaneously. You can switch rapidly between the two environments without rebooting. I’m writing this paragraph on a MacBook Pro laptop, but I’m not using any Mac software to do so. Instead, I’m using the Windows version of Microsoft Word, which runs inside Parallels Desktop.

For this paragraph, I’ve switched back to the Mac side of the computer and am using the Mac version of Word. And I copied the previous paragraph from the Windows side and pasted it into the Mac document. It was fast and flawless.

Each approach has its strengths as well as weaknesses.

Apple’s Boot Camp turns the Mac into a true, no-compromises Windows PC-just like a Dell or Hewlett-Packard computer. When you boot up the Mac in Windows, no trace of the Mac operating system is running; Windows has all the hardware and all the memory to itself. In my tests, Macs running Windows under Boot Camp were wicked fast and handled every single program I tried, including Microsoft Office for Windows, the Internet Explorer Web browser and various games. The downside is that pesky need to reboot when switching operating systems.

Parallels Desktop is much faster than the old software for running Windows on pre-Intel Macs, a Microsoft product called Virtual PC. That’s because Parallels makes use of special virtualization features built into the Intel chips. And it has the great advantage of running Windows applications at the same time you are running your Mac programs. But while quite fast, Parallels can’t match the speed of Windows running under Boot Camp, because it must share hardware and memory with the Mac operating system. Also, Parallels won’t work with certain high-end Windows games, and it can have trouble recognizing some USB drives and CDs. In my own tests, Parallels did handle everything I threw at it, but I introduced only simple games. Everything ran as fast as it would on an average Windows PC, though not as fast as the programs ran under Boot Camp.

Because Windows is much less secure than Mac OS X, when you run Windows on a Mac, you have to take the same precautions you would with a standard Windows PC. That means you need to install and run both antivirus and antispyware software that is generally not needed under the Mac OS.

Apple’s approach is more secure than Parallels’s in this regard — with Apple’s Boot Camp, Windows can’t see or access your Mac folders or files. That means any malicious software running in Windows can’t erase or damage your Mac files. Parallels, on the other hand, optionally allows you to share folders between the two operating systems; if you enable this feature, you could give a malicious Windows program an opening to damage or spy on the files on the Mac side.

Installing both programs is fairly easy, but it’s a multistep process. With Apple’s system, you first download and install Boot Camp. Then, using Boot Camp, you divide your hard disk into Mac and Windows partitions. Next you create a CD to be used later, under Windows, to install software “drivers” that Windows will need to control all aspects of the Mac’s hardware. Then you install Windows. Finally, you install the CD you created.

Note that Boot Camp requires a full retail version of Windows XP, called “SP2.” You can’t use any other version of Windows, and you can’t use an upgrade edition, which requires an earlier version of Windows to be present on the machine.

Parallels allows you to install any version of Windows, even a very old one. But if you use Windows XP, you will in most cases also need a full, retail version, not an upgrade package. You first must buy and install the Parallels program, which creates an empty “virtual machine.” Then you install your copy of Windows inside Parallels. Finally, to enable certain key features, you have to install a program called Parallels Tools, which is included.

This all sounds harder than it is. In each case, the whole process took me about an hour and required no technical skill.

And it’s worth the effort. Boot Camp and Parallels have turned the Intel-based Macintosh into the only computer that can run nearly every popular software program, whether it was written for Windows or the Mac. On the same computer you can edit photos in Apple’s iPhoto program and check your e-mail in Microsoft’s Outlook — simultaneously, if you choose Parallels. Now, that’s progress.

There are three main reasons for the success of iTunes. First, it is tightly tied to the iPod, Apple’s wildly popular portable music player. The only legal downloads of major record label songs that the iPod can play are those sold by iTunes. Second, it is well-designed, works identically on the Macintosh and Windows PCs, and is easy to use. Third, its restrictions on the use of downloads are comparatively liberal: You can copy each purchased song to up to five computers and to an unlimited number of iPods and burned CDs.

For these reasons, nobody else has been able to gain any traction in the legal market by copying Apple’s model, and that includes companies as formidable as Microsoft, Sony and Wal-Mart.

So Apple’s music competitors are trying something else: a whole different model for distributing music legally. Led by the reincarnated (legal) version of Napster, by RealNetwork’s Rhapsody service and by Yahoo’s new music service, these companies are hoping to win by renting music to consumers rather than selling it to them.

In the download model championed by Apple, the music service functions like a physical record store. You choose a track, pay 99 cents, and you own it. As long as you abide by the restrictions, which are designed to thwart mass copying by pirates, the song will play anywhere you want to hear it forever, with no further payments required.

However, those 99-cent downloads can mount up fast. If you tried to fill up even the lowest-capacity full-size iPod-which holds 5,000 songs-with tracks purchased from iTunes, it would cost you nearly $5,000. (Granted, most people start with music they already own when loading a new iPod.)

By contrast, the rental services work on a subscription model. You pay, in most cases, $180 a year, or $15 a month, for the right to download as many songs as you want for use on computers and portable players. And the newest rental contender, Yahoo, has slashed those fees to $60 a year, the equivalent of just $5 a month. That means you could fill up a 5,000-song portable player for just $60 a year.

This rental model has attracted a solid audience, but it is nowhere near as popular as iTunes — not even close. That may be because the rental model is far more complicated and restrictive than iTunes, and has several big downsides.

The biggest problem with renting is that if you stop paying your subscription, even for one month, all the songs you’ve ever downloaded — going back years — will become inert and unplayable. Rental song files are rigged with computer code that requires a monthly digital confirmation the renter is continuing to pay. Without that, the song files die.

An iTunes user could pay $500 to acquire 500 individual songs (buying whole albums is somewhat cheaper) over two years, and those songs are always hers and will always play. By contrast, a Yahoo user might download 500 rental songs over two years for just $120 in subscription fees, but the songs will become unplayable unless she pays hundreds or thousands more in subscription fees over many years, even if the fees rise.

Also, the rules for rental songs are more restrictive than for owned downloads such as Apple offers. At Yahoo, for example, you can store each song on only three computers, versus Apple’s five. And you can install each song on only two portable devices, versus an unlimited number at Apple.

Oh, and you can’t burn rental songs to a CD. To get a nonexpiring, CD-burnable, iTunes-type song from a rental service, you have to pony up 79 cents a track over and above your monthly or annual subscription fee.

Furthermore, rental services are far more complicated than iTunes to operate. At the Apple service, every song is a 99-cent download you can own, but at rental services, there are different kinds of songs. Some can be both rented and purchased (for that extra 79 cents each); others can be either rented or bought outright, but not both. Some songs can only be “streamed” — that is, they can be played directly from the Internet, but not downloaded, even on a rental basis. And some can be rented, but not streamed. You get the picture.

Not only do the rental services feature different kinds of songs, but they feature different kinds of customers, with different privileges depending on how much they pay per month or per year. Some rental plans allow you only to stream songs. Others let you download, but only to store the songs on computers, not portable players. The costliest plans — $15 a month at most services, $5 at Yahoo — allow you to stream, download and store the music on both computers and portable players.

Another huge downside of the rental services is that the songs they rent — and even the ones they sell outright for the extra 79 cents — cannot be played on the world’s best and most popular portable player: Apple’s iPod. That’s because the rental-service songs are encoded in a format owned by Microsoft, Apple’s rival, and Microsoft software is required to play them on a portable player. Apple won’t build the necessary Microsoft compatibility into the iPod.

So rental users are stuck with inferior portable players that don’t sell well and thus don’t attract the huge number of accessories available for the iPod. Apple estimates the iPod has about a 75 percent share of the total U.S. portable player market, with the next-highest brand at just 5 percent. There are over 500 accessories sold for the iPod, such as customized car mounts and leather cases, and just a few for other players.

Given all that, why would anyone use a rental service? Well, the rental model is better for people interested in sampling a wide range of music without a large out-of-pocket expense. That might make it attractive to curious but cash-poor students, for example. The rental services also have many more “community” features than iTunes does, features that allow friends and families to share music recommendations, see what others are listening to and discuss music. So they may be better for people who view music as a social activity.

But for most people, it’s no contest: Right now iTunes and the iPod are the better choice in digital music.