Couple of quick things to try.
1) Make sure the ATI Catalyst Control Center is not running in the system tray! That costs about 20% of your video card's performance.
2) In Control Panel->Administrative Tools->Services (have to set the Control Panel to "Classic View"), disable "Superfetch" and "ReadyBoost".
3) Unload iTunes if it is installed. It will, not "can", but will, hurt your computer performance as well.
4) Make sure the game is not installed in the "Program Files" folder. In Vista and Windows 7, that is about a 5% performance penalty.