I use Windows 7 64-bit Home Premium.
I'm not bothering with Windows 8 right now. I like 7 just fine, and I haven't liked 8 whenever I've tried it on someone's computer.
I'm debating mucking around with Linux so I can learn more about it (I have a Knoppix live-boot disc lying around), but my hard drive's getting pretty filled up and I don't currently own an external drive; I don't want to partition off what precious little space I have left at this point in time. ;-;