If you reinstalled Windows and do not have your product key from a retail purchase, you're probably out of luck and will need to purchase another license.
If you used Windows Anytime Upgrade, you may be able to get help by contacting the retailer used for the download or Microsoft if you purchased from Microsoft before the program details changed.
If Vista came preinstalled on the PC, there should be a product key on the PC, perhaps on the back, for your Vista license.
You're off topic for this forum, which is dedicated to Windows Live OneCare. You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
-steve