Win7 Enterprise 64 (free, of course. Have to get something out of this expensive college education). Had it for a couple weeks now and really like it. And for reference, it is the same as Ultimate. XP Mode seems pretty cool too, though I have not used it for much. Dual booted with XP for now (probably until summer, as I have a lot of programs to switch over for a full reinstall and I don't want to do that during the semester).