So you installed Vista on top of Vista? Don't understand why you would...But if you have a legitimate DVD which was supplied by your manufacturer, I would phone Microsoft support, explain what you have done. They will then walk you through the process of changing the installation code.
DON'T try this on your own, or you will lose data. If you perform a clean installation using your supplied DVD, all your saved files that are not backed up will be lost.
The phone call is your best option.