If the XP machine can connect to the router you should be able to login to the router itself and then you should be able to see the key in the pages that come up. Normally you are looking at typing 192.168.0.1 , 192.168.1.1 or 192.168.2.1 into the address bar on your browser.
Had to do that with sister in law's Vista machine, for some reason it just decided to dump all the settings so connected to the router via her Wii.