XP pro on here, used to run a Gentoo box which was blisteringly quick but took me months to get working properly. Unfortunately the motherboard died on that one :-( Server is running debian quietly chugging away in the corner, just does its job, rock solid and reliable.
Getting a new machine soon, and that's probably going to be Gentoo again, or Suse if I can't get it running properly in any decent length of time.