There can be several reasons why this is happening, but the most obvious reason is that your video card drivers are corrupt. You could try the following steps:
o Click on "Start I Control Panel I Display". Now select the "Settings" tab and click on the "Advanced" button, Under the "General" tab, select "Apply the new display settings without restarting", Click "OK" twice to exit.

o Reinstall your monitor drivers, These are apt to get corrupt after a while. However, if you're using Windows XP, even if your monitor drivers get corrupted, Windows will automatically detect the Plug n play monitor and reinstall the drivers.

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o If that doesn't work, find out who is the manufacturer of your graphics card. Download and reinstall the drivers of your card, Popular graphics chipsets manufacturers are Nvidia, ATI, and Intel. You can download their drivers from these links:

o There are programs that need to run at 640 x 480, 256 colors, It's possible that they are resetting the graphics settings on your PC, This happens when you start running the application. You'll need to identify the software responsible and uninstall it. If these still don't work, there is a possibility that it is a hardware problem and you need to contact technical support,