Vista mostly, currently. I bought some new hardware a while ago and decided the draw of DX10, though small, was enough to make me give Vista a try. I still don't really see any real need for Vista, but I also see no real reason to switch back to XP.

I've got Debian on another, somewhat older machine. It's a nice distro and has a decent mix of depth and ease of use, at least when compared to the slackwares and gentoos of the world. How long have you been using Ubuntu? I switched from that to Debian after six months or so and didn't find myself with any major problems.