Maybe verifying game cache and/or re-downloading the game would fix it. You could also try installing to a different drive, or run CHKDSK on your drive Steam is installed on. To run CHKDSK, open Computer, right-click your drive, click the Tools tab, and in the Error-checking area, hit "Check Now". Tick "Check on next boot" (or something similar, don't remember exactly), then restart your computer.
-Vgr