An issue with your computer is blocking the VAC system. You cannot play on secure servers.

How do I fix this?

Enable DEP

Data Execution Prevention (DEP) must be enabled to play on VAC secured servers. To restore DEP settings to default please follow the steps below:

• Exit Steam.
• Click the Start button, then 'All Programs', and 'Accessories'
• Right-click on Command Prompt and click "Run as administrator..."
  - Please note if you are running Windows 8 you will need to press Windows Key + X and select Command Prompt (Admin) 
  
  
• In the command prompt, type the following command and press Enter:
  
  bcdedit.exe /deletevalue nx  
   
• Restart your computer.
• Launch Steam and test the issue again. 

Restart your computer

If this is a problem for you on all servers then you can try fixing this problem by exiting Steam and restarting your computer.

Repair the Steam Service

This may also indicate a Steam Service failure.  Please try repairing the Steam Service:

• Exit Steam.
• Click Start > Run (Windows Key + R)
• Type the following command:
  
  "C:\Program Files (x86)\Steam\bin\SteamService.exe" /repair 
  
  (If you have installed Steam to another path, please replace C:\Program Files (x86)\Steam with the correct path.)
  
  This command requires administrator privileges and may take a few minutes.
• Launch Steam and test the issue again. 

Conflicting Software

This issue can be caused by third party software interfering with your game or Steam. Please restart your computer, then launch Steam and the game without running any other third party software.