If Avast has a firewall component, make sure that you have Limewire as an exception...Windows XP also has a firewall...same thing, although you should only have one firewall enabled. If you have two, one should be turned off.

Modems & Routers can also have built-in firewalls. Are your computers different in this aspect?

Good Luck

P.S. This will help us figure out what's wrong: System Details - Help us help you