1. Try verifying the cache of the game in steam.
2. your antivirus of some kind might be blocking the game. (try disabling it and running the game or removing the game from the antivirus blocking things. Live-protection is a bitch.
3. Outdated video card driver or outdated directx version.
I think (2) will solve your problem if not then (3) for sure.
I hope this helped.