My first thought would be that you don't have the latest driver for your video card. This should be on a CD that came with your computer, motherboard or graphics card. If not, download it from the manufacturer.
If you don't know what your graphics card is: If you have DirectX installed. Go to start then run and type "dxdiag". That will show you information about your computer as well as your video card type. Click on the 'display' tab to find out about your card.