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Verifying Game Cache Files (GCF)

I see purple checkerboards, weapon or world models are missing, or experience crashes while playing a game - how can I verify that my game files are installed correctly?

Try completing all of the following instructions in the order in which they are presented. Skip any steps that do you apply to you.

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"
Lanuch 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 exit.

Important: One or more files may fail to verify.

This is normal for most Steam games. The files that fail to verify are local configuration files that should not be replaced as part of this process. You can safely ignore this message.
Warning:

Do not run the verification process while performing other system-intensive processes, such as defragmenting your hard drive or scanning for viruses.

Do not run this process for multiple games at the same time.