With Windows XP MS has finally reached what should be the minimum expected level of performance for a mainstream OS. Previous versions of Windows crashed in almost embarassing ways (sometimes the messages and meltdowns were amusing enough to compensate for the frustration). Win2000 was a lot better, but suffered from treesloth speed bootup times. So finally, after years of waiting, we've met the minimum standard.
Nonetheless, I'd have to go with Linux as a favorite OS. Once Macromedia makes a Linux version of Flash (any news?) I'll make the switch permanent.