I've been having some problems as well I was getting really bad frame rate stuttering in the Dark Carnival campaign twice it crashed after stuttering for a bit before hand once in a min or so and once the 2nd level in.
Had a funny feeling it was to do with sound (it just didnt sound right in game) went to control panel>sound to see if there was something funny there and going into the soundblaster tab crashes the window, no idea why it was working fine the other day.
Anyway changed it to stereo from 5.1 and went ingame did the same and disabled multicore rendering and dropped AA to x2 played the swamp campaign fine (one or two jumpy bits aside) until the finale when the tanks showed up and again the large frame rate stuttering came back. (the tank problem is reported
here)
Going to try Dark Carnival again now to see if it still crashes.
Oh yeah im using ATI 4870 9.10 drivers, X-FI Xtreme with newest drivers, Vista 64, Q6600 all at stock.
EDIT:
My antivirus popped up with this when steam was Decrypting?
Must be a false/positive?
Yeah someone else had this on the steam forums and was told it a false positive.