I am assuming that you are talking about wireless connections for both PC's. I am not quite sure why the vista one was unplugged in order to try the XP PC, unless there was only one available power socket.
I assume also that the XP PC has a wireless card? If it does you could try to delete all reference to the wireless connection and start all over again.
It would be a good idea to connect both PC's by ethernet first to establish a connection - is it possible to take the PC's to the router to do this?
I am assuming that the XP PC was working wirelessly before - if not you could make sure that the wireless adapter is up to date by going into Device Manager.