I have a FX5950 and i recently started to have the same problem. I noticed though that my whole system freezes when the graphic card is trying to run a DirectX or OpenGL application (such as 3D apps). I ran 3DS Max in OpenGL and DirectX mode and it froze in both. That's how I found out. But it does not happen every time.
Here's what I did to try to fix it:
I reformated my hard drive, reinstalled XP on it, newest NVidia drivers, and it still happens. Updated the BIOS on my card, and still no positive result. Next thing I'll try is to install an old NVidia driver and see if that does it. I doubt it though.
Does anyone (please) have an idea what the problem might be? I don't want to replace my card, don't have the money. Are NVidia cards known to have this problem? Thank you very much!!!