I think the problem may be that, under the terms of the EULA, you can install the prog on only one machine at a time.
Did you uninstall it from the old XP machine ? During a proper uninstall you get the option ( I can't quite recall at which point this appears) to either leave the activation file on the machine (in case you are going to reinstall on that machine) or to migrate the activation file to a new machine.

I suspect that you have left the activation file on the old machine, and that as a consequence you have a corrupt prog. on the new machine.

No way round it, unless you uninstall from the XP machine and migrate the activation to the Vista one.