The registered version of TweakVista does require activation. Perhaps that is what you are talking about?
To get the current version of TweakVista onto that computer, you should install Stardock Central onto it, download it on your XP machine, make an archive (right-click, Archive application), copy the archive from your Program Files\Stardock\SDCentral\Backups folder to your Vista machine, install Stardock Central on the Vista machine, and then restore the archive (File/Archive/Restore archive).
When you run TweakVista it will prompt for activation. Enter your details, and then follow the instructions given to email the signature request. You should get a sig.bin file in an return email. Put this file into the C:\Documents and Settings\All Users\Application Data\Stardock\TweakVista (create this folder if it is not present) and TweakVista should run.
Or, just connect the machine for a few moments to download and activate it. Might be simpler.
