Here's what I would do in your place.
I'd download the latest drivers from nVidia (pick "Download Drivers" from their menu, select "Graphics driver", then "GeForce 6 series", then the OS you run):
http://www.nvidia.comI have always used the latest nVidia drivers (from nVidia) on my Dell machines with OEM nVidia cards, and that has always worked well for me. I have never used some crappy, out-of-date OEM version of the drivers.
Before you install the new drivers, go into "Add/Remove programs" and remove the current nVidia drivers.
Then I'd reboot and install the new drivers you just downloaded, then reboot again.
If that doesn't fix it, can you tell us if you have done any overclocking of your graphics card?