@supergt:
In order of efficacy:
1) check you've got the most up to date graphics drivers.
2) validate the game cache (properties->local files->verify), keep validating until it comes back with no errors.
3) Run Steam as Administrator (works for a few)
4) launch with "-lv". This will disable all the gore, but supposedly stops the problem.
I did 1 and 2, now reinstalling as my validation ALWAYS came back as broken.
@Mark A:
Supposedly this is normal, it's decrypting all the files. Leave it for another while and it should come back. Your HD should be thrashing in the meantime, if it isn't, try restarting Steam.