connect the two pcs via crossover. both machines should have network interface cards to plug the crossover into, they run 10-20 bucks. setup a peer-to-peer network. assuming you will be playing over tcpip, set each machine with a static ip in the same subnet.
comp a
ip - 192.168.0.1
subnet mask - 255.255.255.0
comp b
ip - 192.168.0.2
subnet mask - 255.255.255.0
on comp a, open command line and run
ping 192.168.0.2
on comp b, open command line and run
ping 192.168.0.1
if either of these fail, try disabling any firewalls (including builtin xp)