Going to assume you have a CRT, and not an LCD, monitor.
Since the top half of the screen is black, and sounds like it is "folded" all into what may be that bright line in the middle of your screen, I lean toward blaming the monitor.
Sounds like first you ought to plug that monitor into another computer and see if it happens there.
if it does, then ... wasn't it about time you started thinging about getting a cool LCD panel right about now .
In the last few days a strange thing has been happening to my monitor. As I am typing this it just happened. LOL
All of a sudden I get a half a screen black on top picture on the bottom. A very bright line of light seperates the 2 sections. Is my monitor going or is it the video card?????