Daft question first. I take it you have shared the printer and enabled printer sharing in the XP installation? I see you've installed another printer and that one worked, so I suppose it's safe to assume you've done that.
It may be worth lugging the laptop to the printer and just plugging it in. Vista should install the drivers from its internal ones. Then try reconnecting the printer to the desktop PC. The Vista laptop will have lost the location of the printer by now, but the drivers will be installed. If you go to add a new printer it SHOULD I think be able to find all the parts of the drivers necessary for install. If this works you can then delete the original printer in the Vista Printers folder.
Just an idea anyway.