I've been running Win2K for quite a while- love it: Stable, fast, easy to tweak and likes being overclocked (as long as your system is stable)
I just got a WinXP pc from work: changed to the windows classic interface right away, locations and methods of changing parameters are not quite as intuitive or easy, and it seems to be a LOT slower than Win2K. Its a laptop so I'd expect some loss in speed but wow! It is a lot slower than my 900mhz Athlon was at default with 2000.
Gotta go with Win2K all the way!