Ok.
1.) The GeForce 7900GT has TWO DVI connectors. The one you are currently using might be bad. Try switching the DVI cable from your monitor to the other one. (See what Happens)
2.) By chance do you have another DVI cable? IF so, try using it. If not, can you get one and try it?
3.) If you are using a silly DVI to VGA adapter... God help me...