Since you allowed the option, I ticked both.
Windows 7 is more secure, better at a whole host of things notably its wireless support, it's actually usably efficient unlike Vista and it's been around enough so support generally isn't a problem. It also handles multiple monitors much better and doesn't shift all your desktop icons around every time someone loads a game at a different resolution.
Windows XP on the other hand lets me run the VRML / X3D plugins and Java ME stuff that I've needed for the last two uni assignments I've been doing, which don't work as well on a 64 bit Windows 7 box. So there are still compatibility things around, though these are fading fast.
For that reason I'm currently running XP because I genuinely need to, but over easter (since that'll be the end of all uni assignments!) I'll probably reformat and use Windows 7.