As far as I know, there is no way to determine if a Product Key is active or has been active except by trying to activate the product with it. There's no database to check.
Here's the process for a manual activtion. If anyone knows if it can be done, they probably do.
To manually activate, go to start, search box and type in slui 4 and enter. Enter your country and press next. You'll be on a screen with a phone number and a bunch of codes at the bottom. Call the number and wade through the automated prompts until you reach a human. Have your product code handy as it will be requested. Explain the situation but it's the same PC and only installed on that one PC. They should give you a code to enter on that screen to activate Vista. I honestly don't think even they can help you with this.
I hope this hellps.
Good luck!
Lorien - MCSE/MCSA/Network+/A+ --- If this post helps to resolve your issue, please click the "Mark as Answer" or "Helpful" button at the top of this message. By marking a post as Answered, or Helpful you help others find the answer faster.