Well, try pinging the XP machine. Start by opening a command prompt in XP, from the Start menu select run, then in the window that opens type

cmd

and hit enter, that should open a terminal for you, now type

ipconfig

to get the hostname and ip address for the XP machine. In OS X either use a terminal or Network Utility to ping those. From a terminal you can do

replace www.google.com with the ip address first and then the hostname.

Post back the results.