Personally, I run Windows XP, but that's only for the games. If more people made C&C style games for... the name you're about to see, I would TOTALY have this OS...
LINUX!!!!
Oh, if you are to much of a MicroSlave, it's black and white for TUX!!! w00t TUX!!!

And also, you can't forget Rupert and Geeko!
Rupert for Ximian Linux.

(I actually have that plushy ^^)
And here's Geeko for Suse Linux. (Suse is pronounced "Sue Sah" It's German, like Ulrich)

(Err... and the little one on top, have him, too. ^^)
Oh, and yes. I am a self-proclaimed geek. I have a hat that has the Intel logo on it, but says "Geek inside" made by Harvard Square.
One last thing.
FIREFOX
^ click it ^

FIREFOX is a link, just so ya know. If you can't download stuff or don't want it, don't click it. Really.