1st of all, Norton FW & XP FireWall are known to interfere with each other. Disable XP's firewall: go to the control panel, double click on Windoze Firewall, check the disable box.

Now configure your Norton using this guide: *Configuring Norton Internet Security

D-Link DWL-P100 Power Over Ethernet (POE) Adapter is something I haven't seen before. It apparently uses UPnP which is good but only "if" that refers to working with programs. Are you sure you don't have another connection device?

You forgot to do the port test I listed at the top of my last post.