If you happen to be stuck with Vista, Microsoft has released the Performance and Reliability update and Compatibility and Reliability updates that fix multiple major issues with the OS. Of note, the compatibility update has some fixes for problems with graphics drivers, which may improve performance in games.
This arstechnica article sums it up rather nicely-I'm surprised this hasn't seen more coverage.
http://arstechnica.com/news.ars/post/20070807-vista-performance-and-compatibility-packs-released.html
Performance and Reliability Update
Compatibility and Reliability Update