I've found WinXP a lot faster than Win2000 on the same hardware. The bootup on XP is much shorter than 2K anyway. I've turned off the big chunky kiddy interface however.

Longhorn is going to be a few more years before it comes out according to rumour. So the next one after that is going to be a while!
And when it does come out, nothing else will be able to read it's new FS format (WinFS). It's hard enough getting NTFS write support going in Linux.