Alternate way to enter product key or activate windows

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',  

 or you can reach to it by Start>All Programs>Accessories> Command prompt

     Right-click the CMD.exe (or command prompt)  file and select Run As Administrator.

1. In the CMD window, type: slmgr.vbs -ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
2. To activate windows after changing the key, type: slmgr.vbs -ato