not having Vista myself yet, I can't quite vouch for this answer but the place I got it from seems quit profficient (- Windows Vista help
Follow these steps:
1. Click Start, right-click Computer, and then click Properties.
2. The System window appears. Scroll down to the Windows Activation section, and click the link to enter your product key. The link might take a few seconds to appear, so be patient.
Hope it works!