There are a ton of things you can do, but the performance gain is minimal.
- keep all drivers up to date
- set your pagefile to a fixed size like 1024min/1024max, and put it on a different drive than the one where you have windows installed
- put your pagefile on your fastest harddrive
- defragment your harddrive(s)
- disable unnessesary drivers and windows services
- close background applications/processes
- turn down audio quality in-game or disable audio
- disable antialias and anisotropic filtering
- lower your resolution
- lower texture resolution
- get a better cooler and overclock your system )