Windows XP, since it's the best for gaming imo. That said, DX11 is pretty sweet so I'll be upgrading to that once I replace my 8800GTS 512 with a HD5850.
I am using window operating system as it is very user friendly and has nice graphics. Currently i am using window 7 which is just awesome.
Customised Ubuntu LTS 10 It's extremely fast and I can modify it to perfection for my personal needs It's everything an OS should be..
currently using Windows 7. By far, the most low maintenance, user friendly with great eye candy GUI o.s. that I've used. It's interface is so cool that I think it's even better than Mac OS X.