If Windows is running slower after you exit out of a game.. That could mean you have a memory leak. In which case you should really check and make sure all your applications that are actively running are the latest versions. If your CPU is being used up during game play, it is apparent adding more CPU power will increase performance.
If you say CS uses physic's, that would have a lot to do with needing more CPU power. In which case you should upgrade your processor.