It might help to check whether you are using the latest display driver
for your card that is compatible with your version of Windows 7. Also check if there are any bios or chipset updates available for your system. Enable AHCI if appropriate (i.e., you have a SATA drive).
Also, not the cause of your problem, I'm sure, but it couldn't hurt to defrag your hard drive.
Check your event viewer, application tab for errors. It might provide some clues as to what the problem is. Check power settings and make sure they are not limiting your performance (maybe try the high performance setting).