I use Windows XP myself at the moment, although in the long run i've used everything from MSDOS6 & Win3.1 to WinME (EWW!). That is, of course, excluding the OSes of my older systems, which started all the way back in my childhood with my very own Commodore 64. Hey, 64kb of memory - WOW, we'll never use that much!
Just for the record, this server runs Redhat 9.0 with Plesk 6.11 controller installed. WinXP may be an absolutely excellent consumer OS, but when it comes to servers, you just can't beat LINUX's stability...