Problems Playing On-Line!!!(HELP)

It's not me that can't play online, it's my friend. Well, anyway, it only let's him join a few servers, mainly esforces.com servers. That's it. Everytime me or him makes a game I can't join his and he can't join mine. I can join all the servers(almost) but he can't do you know the problem? He said he doesn't have a firewall up, but who knows...

if he has XP, he has a built in firewall,
i had this problem with one of my friends, he could find my server but i couldnt find his, so he played around with his firewall ports *DONT UNLESS YOU KNOW WHAT YOU ARE DOING* took a few trys and i finally found his server, have you tried giving him your ip for a direct connect?