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  You can also try the Command Line to change the Product key:

1. Click on the Start Button and type: CMD (into the 'Search programs and files' field in the Start Menu)
2. The Start Menu will show 'CMD.exe', 
3. Right-click the CMD.exe file and select Run As Administrator.
4. In the CMD window, type: slmgr.vbs -ipk <type your product key here>
5. To activate windows after changing the key, type: slmgr.vbs -ato