I have a similar card (Nvidia 8800 GTX) and running the latest Nvidia drivers, and no issues, in XP, Vista or Linux. I did however, have an issue with NWN a few years ago, similar to what you describe, and it turned out to be a sound card/driver issue instead of GPU issue.
Post more detailed specs here. Including what build of XP, and the version number of the drivers, I think I can help.