I use XP, I had a customized XP from a friend who had pretty much the exact same gaming rig as me but after an unfortunate reboot I just went back to a regular XP.
I did actually buy Windows 7, but I popped in the disk and it was like "cannot upgrade, you have to reinstall everything" and I was like you know what....I have like 400gigs out of 560gigs full, I really have no intention of reinstalling everything and seeking out my drivers again. I honestly doubt I'll ever upgraded unless I'm really forced to, even then I won't go with a smile.