Re-install the drivers for your video adapter card.  The drivers will be on
the installation CD that came with your system, or on the CD that came with
the card if you have upgraded it, or you can get them from the
manufacturer's www site, or just search the www using the brand name and
model number of the video adapter card.

This might be an easy fix:

If this problem is less than five days old, restore a copy of the registry
that predates the onset of the problem and see if that helps.  The five-day
limit assumes that the computer is booted every day.  If it is not booted
every day, you may have an old enough registry backup that you can use even
though more than five days have elapsed.  Note that any registry changes
made after the date of the restored registry will be lost.

On the Start Menu, select Shut Down and Restart in MSDOS Mode.  At the
Windows prompt, type: scanreg /restore      and hit enter.  Once the
registry restore screen appears, select the date of the registry that you
wish to restore by highlighting it and then click enter.  If you see a
registry named:  Rbbad.cab, that is a copy of the registry that has already
been replaced.