Try using a tool called Sandra,

If you can spend a little money, 30 bucks or so, use a tool called System Suite 5 (version 6 was just released). These will help find any issues you may be having with your hardware.

Also, what type of video card do you have? If you are not sure, go to START, PROGRAMS, ACCESSORIES. From the SYSTEM TOOLS menu select SYSTEM INFORMATION. Select COMPONENTS from the tree on the left side. Select the component, DISPLAY. This will give you the video card name, model and other information regarding your display hardware. Also, note the version of the driver being used.

I ask because it may be a good idea to update your drivers.