Aye, windows -- Vista, specifically, and my only system. Next computer will probably be linux, though. I'd actually kinda' prefer to stick with windows, mostly due to how much more compatible it is with gaming (the rest of the reason is just because I've been using windows systems for a while), but it's reached the point that some of their business practices mean I can't really ethically do business with them.
Question two, I don't really care so long as it's playable. Mind you, my computer's pretty crappy so that's a de facto preference for optimization, most of the time, but it's a practical rather than ideal/emotional preference. I have pretty low expectations when it comes to performance, though -- I've played games pretty comfortably that were hovering around 15 FPS or lower (AI War, in particular, recently). 30 FPS is, like, a good day, and usually anything more is just cake (if I can even notice the difference). Just so long as it runs well enough it's not painful to play and doesn't try to melt my CPU, I'm usually pretty happy.
Question three, I definitely lean towards games that allow for/encourage modding. I don't dislike single vision stuff, but given the historical track record of modding*, I definitely prefer when the option is there. If nothing else, it means I might be able to swing back by the game a few years after I've played it out and find new and awesome things, which is... pretty great.
*And the many very, very awesome things that have been done with it -- having gone through a good chunk of my teens while the WC3 custom maps were flourishing has a lot to do with this attitude.