I have the same problem. I cant host or challenge anyone. Only they can challenge me and only i can join. Its nothing really. just makes it harder for you to play other people. There is a way around it but not without takeing the firewall off. If you have mcaffee like i do click the Red M in the task tray and go to personal firewall tab. After click the disable button it should be in blue underlined letters, but make sure you put it back on or someone could access your computer and stuff of that matter. I would do this but my dad is a comp wizard and put like crazy stuff on it so i cant shut it off like that. One day ill ask him