If you need to change your product key so that you can activate Windows Vista properly, you can use a great command line tool that will help you do this very easily. Just follow this tip:
Click on the Start Button and key in command prompt so that it shows up on your start menu search list. Right click on the Command Prompt shortcut and select Run As Administrator.
At the administrator command prompt, type in "slmgr.vbs -ipk <insert your product key here>"
To activate windows after changing the key, run "slmgr.vbs -ato"
Sourced From :
tweakvista.com