http://store.steampowered.com/hwsurvey
The majority OS is Win7 64bit with near 35% of users (will increase in a few days when the new months stats are posted).
2. XP 32bit with 24% and dropping
3. Vista 32bit with 13.5%
4. Windows 7 32 bit, 11.5%
Out of those options the only OS gaining market share is win7 64bit. Also I should mention these stats can fluctuate as when civ5 was released alot of ppl with crap computers joined steam to play, it skewed the results for atleast a month if not 2. (about 3-4 months ago).
!!!EDIT: Also you may note that the number is from MAY 2009 - MAY 2010, so it does NOT represent current % of dx11 users, Ie the graph is over 6 months out of date!!!