I'm surprised no-one's mentioned just reinstalling Vista - shouldn't take long and should resolve your problems.
Well obviously something still needs to be fixed if it's still having issues. It isn't Vista's fault...XP can and will end up the same way. Worse, actually, since Vista is much better at fixing itself.
If you must get rid of Vista, go for windows 7. XP is old, and nowhere near as secure or advanced as Vista or 7. What you want to do is roughly the equivalent of going back to win98 when XP SP2 was released.
Whilst I agree that 7 is the better choice over XP these days for all modern machines anyway, XP is a lot better than you're giving it credit for there. I've had MANY more problems with Vista not "fixing itself" than XP, and while it's undoubtedly older than Vista and 7 it's still not a bad OS.
I'm not saying go with XP, for the problem here I'd suggest reformatting and seeing how things go after that as I've stated above. If that doesn't work then have a look at 7. It does seem though that now that 7 has come out everyone's moaning about what a crap OS XP is, which simply isn't true. It's a very good OS, it's still secure, will run most things you need and is perfectly usable - just a little dated now for modern use.