Hardware and software compatibility
Microsoft has admitted that Windows 2000 and NT suffered from huge hardware and software compatibility flaws. It paid for this lackadaisical attitude, too: Windows 2000 never really worked with consumer hardware and games. Thankfully, Windows XP fixes much of the problem.

The XP CD ships with built-in support for about 12,000 devices--twice as many as Windows 2000 right out of the box--with other drivers available via the System Update feature, which downloads drivers as part of the installation process. In our tests, formal and informal, XP picked up much of the hardware we threw at it without a hitch, from USB storage devices and keyboards to MP3 players. In some cases, however, we weren't so lucky. XP flagged a newer Lexmark X83 multifunction device as incompatible, so we're holding out for online updates (something Lexmark and Microsoft will have to work out among themselves).

As for software, XP promises to support 1,200 legacy applications out of the box and offers its Compatibility Mode, which checks to see which version of Windows your software needs, then emulates it. Like Windows Me and 2000, Windows XP is not built on a DOS core, but you'll find a revamped DOS virtual machine for running those old 16-bit games that you should have retired by now. However, XP won't give direct access to hardware such as memory and sound cards, which is how old DOS games used to work, so you can't expect every old piece of DOS-ware to work.

But be warned: Some programs prevent Windows XP from installing. If you're running deep, system-level programs such as Roxio GoBack, you'll need to uninstall them before you can proceed. In some cases, you can reinstall these programs afterwards, but this will work only if the program is compatible with XP. And you won't know that until you run Upgrade Advisor on your computer and it tells you the current status quo.

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Checking compatibility
If you have any doubts about whether your system's components and software will work under Windows XP, you could go the long route and check Microsoft's hardware compatibility list, which grows by leaps and bounds, or use Microsoft's spiffy new compatibility checker, called the Upgrade Advisor. This tool will be available from Microsoft's XP Web site in mid-September, and a similar tool, called Compatibility Checker, is included on the XP retail CD. (Microsoft also plans to distribute Upgrade Advisor CDs for free at many retailers so that you can check your PC's hardware and software before you decide to upgrade, or it may offer the program as a 35MB download.)

On our test systems, Upgrade Advisor flagged several common programs as potentially incompatible (including Roxio DirectCD, Norton AntiVirus, Logitech Mouseware, MusicMatch Jukebox, and Norton SystemWorks), and recommended uninstalling some of them. There were fewer hardware issues, but it flagged the aforementioned Lexmark MFD as well as some modems and older monitors. Some turned out to be false alarms, since the hardware worked fine after installation. However, take the Upgrade Advisor's recommendations seriously. We suffered some hideous crashes that we eventually traced to an older version of DirectCD software--which Upgrade Advisor flagged and we kept anyway--that clashed with XP.

Incompatible software? No problem
Software compatibility is a tricky beast, but Microsoft's Compatibility Mode overcomes some of these issues. If a program refuses to run under XP, right-click its icon and select Properties and the Compatibility tab. There, you can choose an operating system to emulate--one that you think the software would run on (Windows 95/98/Me/NT 4.0/2000). Compatibility Mode also offers 256-color mode and 640x480-pixel screen resolution. Once you've set the mode that you think will work, XP keeps track of the settings and runs the program in that mode the next time around. We tested this feature with some old CD-ROM titles from the early 1990s, including some first-release Living Books and corporate databases. The feature worked fine for us.

Alas, XP's DOS virtual machine is less successful, so popular, older DOS games or Windows 3.1 programs that use DOS programming tricks may cause problems. Descent II, for example, won't even install because XP won't let the setup program probe the system for a sound card.

Tell me the difference between XP Home and Professional!