This issue is detailed on the Steam forums. If you want to just follow their solution, perform only step 3. We have found some additional steps have helped some users.
1. Make sure you are running the game from your Hard Disk Drive. This error can happen as a result of having the game installed on an external drive.
2. Run a virus scan and malware scan.
3. This article details a lot of common issues that Steam can run into when launching the program under "Windows Troubleshooting Tips."
4. A registry cleaner can help your computer locate files that might have been displaced by other programs. (CNET provides safe downloads and ccleaner is often effective.)