I'll assume DHCP is enabled on your router, I'll also assume that there is another available IP address to DHCP on your router (the range of IP's is larger than 2), I'll also assume that the cable between the WinXP box and router works..
On the WinXP machine, make sure TCPIP is enabled, make sure that get IP address is "automatic", make sure your WinXP firewall is off..
Once done, click the "repair" button on that network adapter.. See if you can "ping" from the WinXP machine to your router's internal IP address (it's the same IP as the gateway IP on your other machines)..
If you still can't figure it out, I'll have to help you from my WinXP machine tonight -- I'm not in front of it now..