Norton & XP firewall can interfere with each other. Might I suggest you disable your XP firewall & configure your Norton instead.
Go to the control panel, double click on Windoze Firewall, check the disable box.
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Configuring Norton Internet Security (click on link)
isp = internet service provider. The people you pay to enable to be connected to the internet. Some isp's are known for blocking or filtering p2p share programs or just single them out.