I was going to post this exact same thing, and tried all things suggested. Both computer's XP internet firewalls are down. The router is a Linksys 5-port thingie. I can host Unreal Tournament 2003, MS Rally Sport Challenge, F12002, etc. Just not Ghost Recon! I host, and I am sitting there and my roomate does not see my game either on the LAN games OR internet. I even tried to connect him to my router IP which in 192.168.1.100. I am even able to ping his which is 101, but nothing!
By the way, how can I enable AI in Coop mode?