Windows XP, Vista, 7, Windows Server 2008, Linux, Mac OS X 10.4.11, OS X 10.6.4, and a few legacy operating systems, such as Windows 95.
EDIT: To clarify why I use so many OS's, my uncle runs a computer store and drops off stuff sometimes, like a modded server to run Crysis 2 DX11 max settings, hence Server '08, sadly, the processor's heatsync broke. the 10.6.4 is what my parents use, I dualboot 10.4.11, and the old computers I browse thrift stores for, one time I found a copy of Lego Island in an old Dell's CD tray. What fun.