The times I have seen this sort of thing its always been caused by one of three things. The motherboard drivers were not installed from the driver disk. The graphics card drivers are either not installed properly or corrupted. The DirectX is either not installed or corrupted.
Quite often its the first one. People forget to install the motherboard drivers and lets windows load its own standard driver which may or may not work properly.
I am wondering if perhaps the graphics card has developed a fault. I had a 9800 Pro and it worked fine for the desktop, word internet etc but as soon as I played it game it crashed. It was faulty and was replaced under warranty.
Has your graphics card got an additional power supply and if so have you tried another connector in case it may be faulty?