After you installed it, did you configure NIS in relation to your "Home Network"? If you use a network card to access the internet, that's your network, even though there's only one computer.
Don't know whether this is the answer, but you can try if you haven't.
I don't use NIS 2004, but have Norton Personal Firewall 2003. For the firewall, you should be allowed to configure it. See if there's a tab for Home networking. Select the tab, and if there's a "wizard" button, click that, and just follow the prompts, and accept the defaults. NIS should detect your network connections and allow communications through.