What OS do you use?

Right now im running windows xp since this isnt my computer, and I dont want to repartition the disk since ill loose all my mom's data, but I kinda wanna install openBSD and compile mad shit off the ports tree.

I don't use an operating system.  My old computer has a machine language monitor built in and any time I want the computer to do something I write an assembly program real quick and use BIOS calls.  Sometimes when I'm feeling raucous I'll write my programs down, but only on rare occasions.