Old motherboad fried up

My real concern is that as the old HD contains ALL that was in the old computer and if if now put it to another for the transfer does this create some sw contradiction as there would be two OS for ex. Would it be advisable to boot from the "old" HD for the tranfer time being? ...and does the XP need some sort of app to undertand what's going on when Vista starts to suck up stuff?
Tried to look MS own website, but all this was not really clarified...