Windows XP SP3.
My DAW isn't attached to my network or the internet, so I don't have viruses and whatnot that to worry about. My DAW of choice is Sonar 8 PE, though I do have FL10 and Cubase SX3 installed. The main reason I use XP is because of my audio interface/control surface: A Tascam FW-1082, which won't work with anything past Vista (and I've considered re-building my DAW with Vista). My keyboards and drum pads, an M-Audio Axiom 49 and 25 (1st gen) and OG Trigger Finger also work best under XP mainly because the editor, Edison, works best under it (and neither have drivers for anything past Win7).
I do have stuff that will work with newer OS'es: I have an M-Audio Axiom Air 32 and a Lexicon Alpha interface in storage (I also have a Tascam US428 in storage, but that only works with XP or Vista), but the setup I have is where I'm most comfortable.