I have my P233 running a very trimmed down Mandrake Linux 10.2 (mandriva) (it's chugging along nicely, actually). My P4 runs Windows as it's also my main rig. I do dual boot with gentoo.
I have five running WinXP counting my work computer and a couple of other borgs. It would be six but I haven't gotten my ass in gear to get my dually PII up and running again. Then again, maybe I'll put Win2k on it just for something different.
Almost everything I have is windows 2k or xp. Since they're all at the office I have little choice. My home garden has 2 XP, 2 BSD, 1 Linux and one win98.
My 2 folding boxes both run XP Pro, as I do work on both of them. One is my primary rig, where I game/surf/do homework (when school is in session) and the other is my lab workstation for the internship I'm currently on.
Just a thought, though, you might want to change the poll if you do another one of these in the future. A lot of people have more than one folding rig, and it's rather likely, especially on the [H], that those rigs don't have the same OS.
Right now I have 2 on Windows XP, 1 on Windows 2003. I just ordered a new mobo and proc so I will end up with 2 on Windows XP, 1 on Windows 2003, and 1 on Linux.