You cannot use same connection port for two programs.
First thing to do is set up a static ip address:Static ip address for XP

Then Port Forwarding Limewire on the Motorola SBG 900

Oh in regards to Norton firewall, you need to re-add LW or Phex to it each time you install a new version.

BTW check your internet account online. See if there's any possibility of unblocking ports. The fact you found port 6346 blocked suggests the ISP is responsible for it. You might even need to call them & ask them to unblock certain ports.