NIS firewall has been known to malfunction. If I were you, I would say.....
Norton v. Router ----> Go router
Router v. Other Software Firewalls ---> Toss-up
I would suggest using the router, and then the Windows Firewall in XP (if you have XP). That would work. Or toss the router alltogether and get some other firewall, like ZoneAlarm or something.
I've seen conflicts with NIS a lot lately, in the tech support I do. It conflicts a lot with SP2, but I haven't seen many others conflict. this is just from my OBSERVATIONS, not a take on NIS officially
Hope that helps guide you. Others are welcome to give their take.