How to fix the error message: 'This copy of Windows is not genuine'
To enter a (new) key you first need to follow the steps below:
1. Click on Start button or Start ORB and go to All programs -> Accessories.
2. Now right-click on Command Prompt shortcut and select "Run as Administrator" option.
3. Now run the command slmgr.vbs /rearm
4. The Message Command Completed successfully should show after a few seconds.
5. Reboot the computer and enter you product key