Sounds like a routing issue. Many times this is tripped by the SPI Firewall on the router. Easy way around it is to just disable the firewall, but you will want to have a local firewall setup if you do this (ideally something friendlier then the Windows firewall, like ZoneAlarm or Comodo firewall perhaps). If you keep the firewall active on the router, you will either need to manually setup the port range(s) to forward to your system, or setup UPnP. UPnP will have it's own setting for enable/disable in the router config. You'll need to consult your router's documentation for all the details on where it all is, but it is usually nested in the Advanced settings when you sign into your router's management page.

I think the XB360 is UPnP compliant by default, and should takeover from there--if not, you may need to go into the network config to toggle the service on/off.