Go to control panel>system>hardware>device manager and find your display adapter. Right click on it and choose properties. If it is disabled, choose to enable it. There is also a troubleshooter there to help you find problems with the device. It is possible you will need to update the device drivers, in which case see
Vista here and
xp here
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
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