It sounds as though the driver is corrupt! If you have a separate driver for the display then re-install. If the driver is within windows itself, click on search for better or newer driver. This should cure the problem.
If you are not sure how to do this, please post back to this thread and one of the forum members will talk you through the procedure.
A couple of details would be handy though, Type of monitor and screen size, name/model of display driver. Again if your not sure of the latter then the make/model of PC will help. j.