Except for whatever copies of Windows are still on store shelves or installed on computers sitting on store shelves, you can no longer buy Windows XP after today.
But you can still get XP for new computers, if you're willing to jump through some hurdles. Here is InfoWorld's guide to getting XP now that new licenses are no longer generally available.
[ Explore the issues and passions that drove 200,000-plus people to ask Microsoft to keep XP alive. ]
Get a new "white box" PC instead of a brand-name one. System builders -- typically small shops that assemble PCs from components for you -- can continue to sell Windows XP licenses until Feb. 1, 2009. These Windows XP licenses are good only on the computer you bought them with and are not transferable. Note that the definition of "system builder" is a bit broad, so you may find some companies that will sell you an XP license with a new motherboard. Resellers such as MacMall and CDW that sell Macs bundled with Parallels Desktop or VMware Fusion count as system builders, so you can get XP-equipped Macs this way.
Buy a "downgrade"-ready PC. Most major computer makers -- including Dell, Hewlett-Packard, Lenovo, and Toshiba -- will sell at least some models for which XP replaces the Vista license that came with the machine. Microsoft lets computer makers sell Vista-equipped PCs and then offer a "downgrade" to Windows XP Professional. (Sorry, not to XP Home.) Essentially, the Vista license covers the XP installation, and you can reinstall Vista later under that same license. The PCs will come with the Vista discs, as well as XP discs or reinstallation partition. What's unclear is how long the computer makers will offer this option; Dell has committed only until February 2009, for example. And don't be surprised if you have to pay extra for the "downgrade" privilege. Oh, and don't expect to find these in physical stores like Circuit City and Best Buy; you'll find them almost exclusively at the computer makers' online stores.
"Downgrade" Vista yourself. If you buy Windows Vista Business or Vista Ultimate -- whether preinstalled on a PC or as a shrink-wrapped full version or an upgrade version of the software -- the Vista license lets anyone "downgrade" the Vista OS to Windows XP Professional. (Sorry, not XP Home. And other Vista versions cannot be "downgraded.") But you must supply your own XP installation media. Note that any business or enterprise that has a Vista site license also has these downgrade rights.
Buy an ultracheap PC or laptop. XP Home will be available on ultra-low-cost laptops and desktops until 2010. The Asus Eee PC is the best-known example of this class of under-$600 system designed for poor countries but also available in rich countries like the United States. They simply cannot run Vista, so Microsoft is allowing sales of XP on them until Windows 7 comes out.
Scour the Internet for leftover XP discs. You'll find old copies of XP available through various online retailers. Just be careful that you know what you are getting: They may be pirated copies, or previously licensed copies you won't be able to get validated. And chances are many will be system builder or OEM licenses that are technically tied to a system you don't have and are not transferable to your computer.