Windows Vista, 7, or 8 users
Skip this step and proceed to the next one if you do not use Windows Vista, 7, or 8.
Completely exit Steam
Right-click on the shortcut which you use to launch Steam
Select "Run as Administrator"
Launch Steam again and continue to the section of this article titled "Verify integrity of game cache"
Windows XP users
Skip this step and proceed to the next one if you do not use Windows XP
Exit Steam
Go to the "Start" menu, select "Run", type "regedit" and click "OK"
Navigate to the following registry key: HKEY_CURRENT_USER\SOFTWARE\VALVE
Right-click and select "Permissions"
Check the "Allow" boxes for "Admin" and "System" for both "Full control" and "Read".
Launch Steam against and proceed to the section of this article titled "Verify integrity of game cache"
All operating systems - Verify integrity of game cache
Load Steam
From the Library section, right-click on the game and select Properties from the menu.
Select the Local files tab and click the Verify integrity of game cache... button.
Steam will verify the game's files - this process may take several minutes.
Once the process is completed, the Check Window will automatically exi