You'll need to install the drivers for your video card. If you don't have the drivers on a disk that came with the computer (or the video card if you added it later), you can download them from the card manufacturer's site. You'll need to know the make and model of the video card you have.
To find the make and model of your video card, download, install and run either
Belarc Advisor or
Everest Home Edition. Either one will inventory your computer and give you a lot of information about your computer's components including the "Display Adapter" or "Video Adapter" (which are just other names for video card).
Once we know the make and model you have, we can help you find the drivers for it.