Overture (Steam) crash

[Thoth]

Steam version of Penumbra: Overture, running on Vista 64
Video card is an ATI Radeon 4870 (512mb)

I've been encountering scattered crashes on loading screens all day (just bought the game today), and up until now I've been able to get through them by closing the game down, restarting, and trying the door again.

Now, however, Overture crashes every time I try to load the next area (the keypad-locked storage area door from Iron Shafts B to Iron Shafts C).

This is the error message I get from BlackBox:

Exception Reason:
penumbra.exe caused an EXCEPTION_ACCESS_VIOLATION in module atioglxx.dll at: 0023:6906EB99, DrvPresentBuffers()+18857 byte(s)

Stack Trace:
0023:6906EB99, DrvPresentBuffers()+18857 byte(s)
0023:690BA621, DrvPresentBuffers()+328753 byte(s)

I have tried reinstalling, changing audio and video settings, and all the other usual fixes, but nothing has made a difference.

Please help. I have really been enjoying the game so far, and would really like to be able to finish it!