Re: Windows 7 vs XP
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.
Save the whales, feed the hungry, free the mallocs.