nVidia released a new (beta) Forceware driver that fixes some incompatibilities with the 6600GT and some motherboard chipsets.
Release Highlights:
The issues that some users have reported seeing when using the GeForce 6600 GT AGP are due to a driver bug which causes 3D corruption when this GPU is used with certain motherboards, most notably SIS motherboards. The fix is included in this release.
ftp://download.nvidia.com/Windows/67.03/