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.