It seems to be a problem with your display adapter.
Go to device Manager and uninstall the display adapter. Your computer will prompt you to restart.
After restarting the computer, insert the display adapter drivers which are provided on your CD that came along with the AGP card or video card (whichever you have)
and then install them.
Then this might resolve the problem. If it doesnt then let us know how long has this problem been going on and you might try to use System Restore to restore the system to a prior date when the system was working fine.