Here is two command lines to change a Windows product key (Vista,  7 or  8) and activate Windows. They must be executed in a Command Prompt “Run As Administrator”.

1. To set a product key, run “slmgr.vbs -ipk <your product key>”
   • Ex.: slmgr.vbs -ipk 00000-00000-00000-00000-00000
2. To activate windows after changing the key, run “slmgr.vbs -ato”