reply to chess This is out of left field but have you messed with your router settings? There is a setting in your router called Enable UPnP which the xbox depends on to open ports. I do not have an xbox and turn this service off as it is a security risk.
The use of it is that any program on any device on your network can poke a hole in your routers firewall
While it is nice that a trusted device like your xbox can open ports so you can set up games with friends (as your average user has no idea what a port is) but that means any maleware can go to town on your router as well.
