1. Go to your Boot Camp .app in Applications/Utilities and remove your existing BC partition.
2. Resintall Boot Camp (be sure to resize your partition appropriately...you don't want to have to do this again, do you?).
If anything, when you reactivate Vista, it'll ask to call Microsoft's toll-free number (assuming you live in the US) and will go throuhg a series of challenge/response codes with you. NOTE: be sure to state that you have Vista installed on only ONE system. Anything else and you'll either be denied a new activation code or you'll have to go through the hassle of speaking with a Microsoft rep.
Best of luck!
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Unibody 15" MBP | 4GB | 320GB 5200 RPM HD | OS X 10.5.6 | Microsoft Office 2008 for Mac | Boot Camp w/Vista SP1 | VMWare Fusion 2.0.1 | 1 TB Time Capsule