Follow these steps to change your product key in Windows Vista.
Note: If you bought an edition of Windows Vista other than what's installed on your PC—for example, if you purchased Windows Vista Home Premium, and Windows Vista Ultimate is now installed on your PC—you'll need to perform a custom installation to update your product key.
Important: You'll need your new product key to complete the product key update process. For examples of where to find your product key, visit the How To Tell website.
Scroll down to the bottom of the window that appears, and then, under Windows activation, click Change product key. If you're prompted for permission to continue the process, click Continue.
Follow the instructions to change your product key and activate your copy of Windows Vista.