Did you try a System Restore?
System Restore
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You might try a System Restore to a point prior to your problem:
start > All Programs > Accessories > System Tools > System Restore
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Otherwise things are not so simple. Sorry.
If your computer is a brand name and you never added any hardware to it then tell us the make and model of the computer such as: Dell Dimension 1234 or HP Pavilion 1234 etc.
If on the other hand you or someone added a graphics card to the computer we need to know the make and model of the graphics card.
You could check under: "start > control panel > add or remove programs" and see if you have any programs listed under either ATI or NVIDIA.