To change Windows 7 Product Key From Command line.

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 {product key}
5. To activate windows after changing the key, type: slmgr.vbs -ato