From the other PC's (excluding the XP machine), when you connect to the XP machine, are you able to see the shared printer? Have you tried deleting the printer from the other machine's and reinstall them? Also after deleting the printers, I would run ccleaner and make sure the registry entry's are cleared as well... Then connect and reinstall the printer.
Also, any error's when trying to print? If so what do they say?