Off the top of my head, both machines have to be in the same subnet (255.255.255.0 for example); also I would make sure the ip group (192.188.1.x where x is different number from 0-254) and workgroups are the same as well. The workgroup may not matter but these machines should be on the same network to work. You can use 'Directory Access' in the utilities folder to change to the same workgroup. Once your machine and XP are on the same network, iTunes should find the library via Rendevous.
Terrance