Make sure that the Windows firewall's configured to allow LW.
User Cant Connect - LimeWire

A few people have had problems with Java 1.6.0_01. Try this version instead.
Use the offline installer& remove your current Java version before you install this one.
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11

If that doesn't help, delete LW's preferences folder when LW's properly closed. See point 3 here.
Fixes for LimeWire
This will kill all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through LW's tools > options & write them all down, so that you can reset things later.

Try port forwarding for your router. The instructions are here.
Port Forwarding for the Linksys WRT54GS
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 a router all have to be the same number