If you're using Norton's firewall then make sure the XP one is disabled... the 2 won't work properly together. Make sure your Norton firewall's configured properly
Firewall setup

Go here with your browser, see if the test works
http://www3.limewire.com:6346

LW will detect a modem/router's internal firewall, too. Have a read through this thread, there's a lot of info about enabling UPnP or port forwarding to overcome this problem. Try enabling UPnP in your device, see if that helps. If not then you might need to port forward.
Connection problems? Check here first!

If you're still stuck, post back with your details

OS(Win XP etc)
LW & Java version (help > about LW & you'll see)
Firewall you're using
Modem/router brand & model
ISP
Connection type (DSL etc)
Where you connect from (home, school etc)
Are you sharing a connection with other computers
Is this your first try with LW