Two possible ways:
- If your WRT54G supports a static route on the WAN side of the router, setup a static route to 192.168.100.1 and it should work.
- Add a hub (or switch) between your modem and router WAN port. Add an extra CAT5 cable between the hub and one WRT54G LAN port. Configure your XP or W2K box for a 2nd IP address on the ethernet. I scribbled something on the topic a while back: formatting linkof 10.0.0.xxx in my example, use 192.168.100.xxx instead.