Your Netcomm's probably got an internal firewall. What model is it (there's NB1300plus4, NB5plus4, NB6Plus4W)??

Have a look at the router list here. Find your model, choose LW from the list of programs & you'll find instructions for port forwarding...that fixed mine
PortForward.com - Free Help Setting up Your Router or Firewall

Make sure to set up a static IP address first, else port forwarding won't work. You'll find links for that within those p/f instructions.
LW's listening port, port in the 'manual port forward' box & port forwarded should be the same number

Did you disable the XP firewall & reconfigure Norton?