I had a similar problem, and it was not a heating problem.
Make sure you get SP 2 (A clean install of XP had odd effects on NWN, so I upped it to SP 2), and patch it to the max. Also make sure you have DX 9C (
http://www.microsoft.com/windows/directx/default.aspx)
Also if you are using the beta NWN patch that might be the problem, a lot of people have been having odd problems with it.
For your nVidia card, are you using your manufacturer’s drivers or the nVidia drivers?