Much lower framerate since upgrading video drivers

I upgraded my video drivers to the latest version so I could play Bioshock, and since then I've had a horrible framerate in SL. Other than installing Bioshock and the video drivers, nothing has changed on my system. I've got a dual-core Athlon64 4200+ with 2GB RAM, a GeForce 7950 GT with 512MB RAM, and I'm running Windows XP Pro. Since the video driver update, I get around 1/3 of the framerate that I get on an older machine that's running Linux (Single-core Athlon64 3600+, 1GB RAM, GeForce 7600 with 256MB RAM, Debian Etch). If I turn on OpenGL Vertex Buffer Objects, the frame rate increases, but it's still less than my Linux machine's, and it stutters badly whenever I rotate the camera, sometimes badly enough that it locks up entirely. I don't really want to bounce back and forth between drivers to play different games - what caused the big slowdown with the latest drivers, and what (if anything) can I do to fix it?

Thanks,
Warin

Are you sure you uninstalled the old video drivers completely? Including using a driver clean up program after (Like driver cleaner). Kind of sounds like you didn't.

I installed the old drivers completely, including my desktop profiles. I've never needed a driver cleaner before (in fact, I've upgraded my drivers a few times since starting to play SL, with no slowdowns before).

I have now uninstalled the drivers completely (again), run Driver Sweeper, and reinstalled the drivers. No speedup; I'm still getting a much lower framerate on my gaming machine than on the machine with a slower CPU, half the cores, half the system RAM, and a lower-end video card with half the video RAM.

what are exact driver version numbers

6.14.0011.6371