Retail sellers are prohibited by Microsoft from selling WinXP already, unless they have "existing stock." Most existing stocks are gone now.
Some OEM makers have existing stock (licenses for specific quantities) that they can continue to deliver, but for the most part they're only willing to deliver them to "business accounts" who buy machines in quantity (or sometimes to those who beg very convincingly).
Microsoft has made claims about "keeping XP available," but the only versions being kept available for new users are XP Home (which is completely gutless for my needs, and is available only to specific "kinds of users"), and the de-watted neutered version(s) for game toys.
Support for XP, except for critical security patches, has ENDED, so you're at the mercy of Microsoft if anything comes up that they don't consider "critical." The sleeper in the support situation is that they shouldn't be expected to check patches they do release for compatibility with older application programs, so those are likely to begin to deteriorate.
Critical patches to WinXP rendered my perfectly satisfactory Word 2002 unusable about a year ago, and the only thing available then was Office 2007.
They mucked up Office 2007 even worse than Vista, in my opinion. I was perfectly happy with FIVE choices on the top tool bar, and could get to a menu for any task with a maximum of about three clicks, if I forgot the shortcut keys. Now there are 53 unintelligible and unusable choices on the default top menu, including 7 other "main menues" each with about the same 50-odd unintelligible and unusable choices to do things I don't need or want to do. Seven clicks down, you can get to half of a useful menu, but the other tasks that used to be all together in one place are now splattered randomly among the 8 X 50 x 50 menus you have to click through to find where pieces of useful menus might be hidden.
Of course that's just the front page. If you want my real opinion it would take several posts.