What version of the nVidia Detonator drivers are you running? What OS?
Most times if you have a TnT level card, the newer drivers won't work very well. I'd try the 7.58's for Win2k/XP and the 7.78's for Win9x/ME. If that doesn't solve it, start stepping up through the WHQL certified drivers until something works. With that card though, you'll want to stay with much earlier versions of the Detonators.