Windows Vista product key can be changed using the System settings. Click on the Start button –> Navigate to Control Panel
select System from the list of available options. Scroll down to Windows activation section.
If your Windows Vista is already activated then Windows is activated message will be displayed along with the Product ID. If you want to change the product key then click on the Change product key link. The following Windows Activation dialog box would be displayed. Enter the new Product Key and click Next to Continue with the Operation.