Can the machines ping each other?

You don't really have to run Norton Internet Security internally if you are using a cable/DSL router like the RT314. I'm assuming it is similar to the LinkSys in that it has a hardware firewall of sorts that will not let anything into your network unless you explicitly allow it...

Have you tried disabling Norton completely? I have not used the application so I'm taking a stab here on the network side but like MightyPete said, Norton is pretty intrusive and is probably the culprit (assuming both machines can ping each other and it's not a routing issue)

Zone Alarm (free) or BlackICE ($$) are better alternatives to Norton but like I said, if you are using the router - your internal machines are somewhat invisible to the outside world...