multiplayer - a new member can't connect to my computer

Hi,
I use the multiplayer to fly with my father. There are no prblems. Usually I host him after giving him my IP.
Yesterday I installed FS2002 in my friend's computer and tried to join him to our game. I did the same steps as I had done always, only he had all the time a disabled "join" button in the multiplayer window. He tried even to declare himself as a host but we couldn't join him as well ( we had the disabled join button that time). I shut down the Norton Antivirus and he closed the Macaffe application but nothing has worked.
we have reboot our computers to swich to another IP (that is given by our Internet provider) but nothing has worked.

Do you have any more suggestions?

Thank you.

Pampela

If either one of you is running a firewall or a router that's your problem.

Thank you but I turned off the Macffee and Norton on both computers and still we can't recognize each other.

Those are anti virus's software I mean a firewall, unless you bought their firewall software also.

Check this.