Xp pro on my main machine, XP home on the laptop, 98Se on 5 other machines, and in a removable HD bay on a test machine here, I roll in and out W2K pro, W2K server, WinME, Mandrake 8.1, and XP pro dual boot. I have Small Business Server in the garage shop hooked up and I am evaluating .net server RC2 on a new Hdd  in a few days.
I am somewhat overwhelmed by all this technology, but Microsft XP is the best damn desktop I have run so far.
I  think .Net server will be the best server product too. They have taken everything you didn't like about W2K out and pur in everything an admionistrator could ever want, and with XML, it  can manage about any member server out there, *nix inluded.