I see that you've already been given the link to the 'if LW will not connect' thread. I guess you tried everything there?
Have you tried changing your Java version to the latest, & also try the older version 1.5.0_11? Did you make sure that there was only the 1 version of Java installed on your computer? There's now a Java 1.6.0_04...if you haven't already downloaded that then try it. I'm using it & LW runs fine.
When you replaced the gnutella.net file, did you make sure that LW was completely closed first (by checking in Task Manager)? If LW's not properly closed, replacing gnutella.net or deleting the preferences folder is a total waste of time. Did you restart your computer after replacing gnutella.net & deleting the prefs folder?
Go to the gnutalla.net file, hover your mouse over the file...what size (KB) is the file?
Have a look at the post below...even though it's really old, a couple of people have recently found that it's helped with their connection probs.
http://www.gnutellaforums.com/78282-post22.html
Also try changing ports. In LW tools > options > advanced > firewall config. Change LW's listening port to 65000 (or try any ports in the range of 49152 - 65535) > apply > ok.
There's no firewall within your modem, so port forwarding's not an issue. AFAIK there's no UPnP support either.