Norton Firewall and Armagetron Advanced
The problem is that Norton Firewall's policy for outgoing UDP traffic is twofold. The global policy is to reject all outgoing traffic. Then, as soon as an application starts to use UDP, Norton Firewall creates an exception ruleset for it, allowing it to use UDP traffic. At the very least, this process takes time, and initial communication of Armagetron with the master servers and servers fails at first. But for some people, the exception rule never gets applied, or something else goes wrong.
The Solution (for Norton 360, other products should be similar)
Maybe you don't even get a problem with Norton and Armagetron Advanced; for many people, the exception rules should eventually apply and the game should work normally. Of course, those people would not read this page at all, so for those who get their game permanently blocked, here are your options:
The easiest solution is to uninstall Norton. The author would never let bloated sluggish cargo cult software like this on any computer near him, so this is probably the best solution. Of course, you should get a replacement for the virus scanner and the firewall. For the latter, the firewalls of XP and Vista are just fine.
The next easiest solution is to just disable the firewall bit of Norton and replace it with the XP or Vista firewall or any other firewall of your choice. To do that in Norton 360, open the main window (right click on the system tray icon, select topmost menu entry) and select "Tasks & Settings" on the top of the Window. Go on to click "Change Advanced Settings" on the lower right, then "Firewall Protection Settings". There is your off switch. Press it, click on "Apply" and select "Until I turn it back on" in the following selection.
The second hardest solution is to install a global exception that allows outgoing UDP traffic. Go to the "Firewall Protection Settings" as described in the previous paragraph. Select the "Firewall General Rules" tab, click on the "Add..." button. In the wizard, accept all default settings, always click "Next". Only in the last page, you can give the new rule the name "Allow Outbound ALL". To make the rule slightly less generally allowing, you can restrict it to UDP only on the fourth wizard page, there should be a dropdown list for that. Click on "Apply" when you're done.
The hardest solution is to install a program specific exception rule for Armagetron Advanced. Note that other games will have similar problems, and only in this method, you need to repeat everything for every other game as well. To do that, you need to run Armagetron at least once while the Norton configuration windows are closed and try an internet game. Then go to the Firewall configuration as in the previous paragraph, select the "Firewall Program Rules" tab. There should be an entry named "armagetronad". If not, tough luck, go back one paragraph. If the entry is there, select it, click "Modify...", click "Add..." and then follow the instructions of the last paragraph after the "Add..." button click there.
In any of these cases, you need to exit the configuration to save and really apply and save your changes.