With Windows Vista, Microsoft has enabled users to change the product key without reinstalling the operating system. Lost or stolen product keys can be changed in Windows Vista via the user interface or through a command prompt.
If your Vista product key was a Volume License Key that Microsoft has identified as leaked, with the first WGA update the operating system will be moved into reduced functionality mode. In RFM, Windows Vista will permit users to retype the product key. This is the first method to change the Vista product key.
From Vista, users will additionally be able to Run Elevated Command Prompt with administrator rights, by right clicking on Command Prompt in the Start Menu. Alternatively, you can simply enter the text “command prompt” in the search box integrated in the Start Menu and right click the shortcut and select “Run as Administrator.”
Enter slmgr.vbs -ipk <> in the command prompt. Be sure to replace <> with your actual product key. In this manner you will change the product key of the operating system with the new one. After performing the change, you will have to activate your copy of Vista. You can do so via the “slmgr.vbs -ato” command inserted in the command prompt window.
Another way is to navigate to the System Properties via Start Menu, Control Panel, System and Maintenance and System. In the Windows activation area towards the bottom, you will be able to enter a new product key via the “Change product key” option.