d4a,
Sounds like the same problems I had with G2 when I upgraded my nvidia drivers a few months ago.
After hours of screwing around with G2, I removed the entire G2 program and add ons from my machine.
Re- upgraded the newest nvidia drivers and reinstalled G2.
Then went to online update at G2 (before I tried to run it again) and downloaded the newest version/upgrade (free to registered users) and voila!

Lots of work, but problem solved.

Jeff