I've never seen this issue before, this is a new one to me. Aside from calling tech support or PM Lycan or Rufioh, Have you done a factory reset back to default, put one PC on the router and connect up to ur ISP WAN? Save ur current settings before doing this. Don't change anything after the reset, just see if it's reproducible after a reset. On ur XP PC, make sure the LAN and Network settings are setup for DHCP and not static manual addressing. I can only think that maybe the IP address is static on the XP PC and is conflicting some how with something else on the network or the router. Make sure all is clear and set for DHCP.
Leave wireless off. Then if all is good, go back and starting adding reservations and adding PCs and see if this crops back up. This is odd. Might try and PM these guys too. Its the weekend so might not get anything until monday. Let us know if this works and keep in touch. Will try and help if I can.