Put a network card in both (if they don't have one) and connect them with a crossover cat5 cable. If they're too far apart for that, you MIGHT be able to put a wireless card in the XP machine and configure an ad-hoc wireless network to the 98 machine, with two IP addresses assigned to the 98 machine's wireless card (assuming it's a standard wireless card). I don't know for sure if wireless cards can be given multiple IPs and networks.
What does the 98 machine connect to for wireless access?