http://www.sizzledcore.com/2009/10/26/how-to-change-windows-7-product-key/
You can also try the Command Line to change the Product key:
- Click on the Start Button and type: CMD (into the 'Search programs and files' field in the Start Menu)
- The Start Menu will show 'CMD.exe',
- Right-click the CMD.exe file and select Run As Administrator.
- In the CMD window, type: slmgr.vbs -ipk <type your product key here>
- To activate windows after changing the key, type: slmgr.vbs -ato
Hope that helps,