I agree with Makaveli213, games make pcs lag when there are many other applications still running. Try lowering the resolution of the game and the graphics. Furthermore, if on Vista you can right click the game shortcut go to the compartibillity tab and select Disable desktop composition (this will disable Aero when the game runs) and run in compartibillity mode with windows xp service pack 2 (who knows?). Another tip for both OS's is to shut down all the other apps when playing games. What are your specs btw, as macdawg already asked?