Again, while I'm more familiar with XP than Vista's handling of this, I think you'd have no risk of a failed activation on the alternative computer damaging the activation status if you were to put the hard drive back in the original computer.

XP remembers the "hardware ID" under which it was activated and is always comparing that to the current hardware to make sure that it's still in tolerance. If it finds itself in an out-of-tolerance state, then it starts complaining, but it won't (so far as I know) obliterate its memory of its proper state.

So... (no warranties expressed or implied) it should be safe to try the brain transplant and see if it works.