Internet Connection sharing can only be enabled on a modem (Dialup, cable,DSL) not on a LAN connection. Windows figures if you have a LAN connection to internet you don’t need sharing, just connect other system to LAN. Options are to get yourself a cheap HUB, and make connection that way. Or (And I don’t know if this will work on XP), Enable routing service on XP box, and specify Ethernet IP of your XP box as gateway for the other system. You may have to add some additional routing information on the XP System.