I just upgraded the CPU (only) on my PC, under Vista Home Premium X64.
I did not need to activate Vista again. (I checked the activation status
just to be sure.)
Your experience may be different, based on other changes that you may have
made in the past. If you try to activate online and get a message that the
key is already in use, use the telephone method (toll-free call, at least in
the USA). Don't bother dealing with the automated telephone activation; you
won't get a different result than the online activation. Speak with an MS
representative, explaining why you need to activate again. I believe that
you'll be given an activation code, even under the most restrictive licenses
(OEM).
Return address scrambled. Replace nkbob with bobkn.