Although the cause of this problem is not fully known, the following 2 steps should fix the problem. In the Monitors (or the Arrange section of Montors and Sound) control panel:1) Take the Video Monitor icon and place it directly on the Computer Monitor icon. (This makes both screens go to half size and shows two little monitor icons.)2) Take the Video Monitor icon and set it exactly on top of the Computer Monitor icon again. (Both monitors reset as normal.)