You may want to reinstall the video drivers. Remove the video card
from the Device Manager and restart Windows. Windows will reinstall
the drivers now. When you are prompted for if Windows should use the
existing driver files on the HDD, vote no and let Windows copy the
driver files from the Windows-CD. If you are using a third-party video
driver, cancel the New Hardware Wizard and then reinstall these
drivers manually. See the manual for the video card for detailed
instructions.
If that does not help, reinstall the monitor drivers on the same way.
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(tm)
You may also wish to update your monitor drivers. I've found this to be
very helpful, as well. If your monitor is a common brand, chances are that
you'll find a monitor-specific driver for it, rather than the current, "Plug
and Pray" monitor driver you most likely have. It's worth a shot, anyway...