You could also try updating your Java. Use the JRE offline installer & remove your current Java version before you install this one.
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11

Here are the port forwarding instructions for your router.
Port Forwarding for the Dlink DSL-502T
Don't forget the step of setting up a static IP address first. LW's listening port, port in the 'manual port forward' box & port forwarded within the router all have to be the same number

You could try reconfiguring Norton's firewall.
http://www.gnutellaforums.com/226610-post6.html
PortForward.com - Free Help Setting up Your Router or Firewall

Were you able to download ok in the past? What's happening now when you try to download? Is it happening with every file or just some?