Does your router provide DHCP? If it does you need to go into your "administrative tools", "services" and kill DHCP in XP Pro. It enables by default and sometimes won't disable when handed off to a router. Try to reset the router, then do a release/renew on your XP box. If DHCP is running on XP and the router, well it makes a mess of things....put very simple.
Hope this helps.....