If the old monitor still exhibits the half-screen problem when attached to another computer, and another monitor works fine with your current video card, a likely explanation would be that you've had a failure in the vertical scanning circuitry inside the monitor.
This is an irritating problem!! my monitor would go half screen once it plays any video file using windows media player, powerdvd or any other software. that is it will be playing in left half and right half is blank!! Once I shake my mouse it will become back to normal. What is the problem ?? is it with my monitor or video card??
Also when it plays games, games will be blurred and cannot display properly!!