I have a Nvidia GTX 295, with the most recent graphics drivers installed on a CLEAN install. The game plays just fine at medium graphics settings until it starts raining. Then the graphics driver crashes every 2-5 seconds. This happens at ANY graphics setting. I've been getting around it by meditating the time away until the rain stops, but I'm at the point in the game where you fight the Kayran and it's just completely unplayable. Is there some adjustment I can make to an .ini file somewhere that disables rain?

You should be able to go into your User.ini file (in the "My Documents/Witcher 2/Config" folder) and turn it off. Under the [Rendering] section, change AllowRain=1 to AllowRain=0.

Surprisingly, that does absolutely nothing. Still rains. However, on a positive note I realized through a bit of research that the Physx driver is interfering with my graphics driver somehow. Using Nvidia's control panel, I set the Physx to use one GPU, rather than self-determine, and the driver crashing has all but gone away.