Serious lag problem

[Mattt]

Hi. I just started playing the full version of the game. It plays fine other than some minor artifacting until I leave the old office (the one early in the game, where you find the explosives manual, among other things). As soon as it loads the mine again, the frame rate is awful, worse than 1 fps. I've tried all of the following, none of which seem to make the slightest difference:
-Apply the 1.02 patch.
-Apply the 1.03 beta patch.
-Turn down some of the graphics options
-Run the game right after booting into Windows, without opening any other apps.

I'm running Windows XP, service pack 2. My system specs:
AMD Athlon 64 3200+ processor, 2.02 GHz
2 gb ram
GeForce 7900 GT/GTO with 256mb ram