Essentially, what you need to do is boot to your Vista Upgrade dvd, install - but *do not* put in the product key, and deselect the "Automatically activate..." check box. Choose 'Custom Install, and on the list of options select the version you actually own - "Home Premium", in this case. Proceed with the rest of the install from there. Again - Don't put in the product key. When you're done, you'll have a temporary install with a 30 day licence.
Then you "Upgrade" the install you just did using the same disc.
When you've completed that, you'll still have 30 days in which to activate. The difference is this time the activation will work: Open the Start Menu, right-click Computer, and choose Properties. There is a link near the bottom where you can 'Activate Now'. Follow the prompts... Done.