1st you'll need to set up a static ip which is why it might not have worked for you before. Without a static ip, everytime you log onto the comp your ip would be different & thus the settings would be useless. So ...

1. URL=http://http://www.portforward.com/belkin/f5d7230-4-dhcp.htm]Setting up a Static IP Address with the Belkin f5d7230-4[/URL]

2. Port forwarding instructions for the Belkin F5D7230-4 Use Port 6346.

You said the only firewall is the Belkin. But XP has its own firewall: http://www.limewire.com/english/content/firewalls.shtml