The gnutella.net file is within this folder - C:\Users\your username\AppData\Roaming\LimeWire(which is hidden by default), so you'll need to be showing hidden files & folder in order to see that file & delete it.

If you're showing as behind a firewall then you could try port forwarding. First of all, you'll need to set up a static IP address for your computer.
PortForward.com - Free Help Setting up Your Router or Firewall

After you've done that, the instructions for port forwarding are here: Port Forwarding for the Linksys WCG200v2

LW's listening port, port in the 'manual port forward' box & port forwarded within router setup must all be the same number