Hello, sorry you are having this issue.
Can I get you to do a file verification in the steam library.
Go to games section and in the left in the Game Guru icon.
Right mouse click on it and press properties.
third tab right bottom button. verify game cache files.
This will go thru the installed files and make sure they are correct. then if some are not then it will re download them.
See if this fixes the issue.
If not then we will try the next step.
I am also going to move this thread to the bug reports section.
Thanks
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit