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.

http://nzone.com/object/nzone_downloads_winxp_2k_67.03.html