Have you read through this sticky?
Connection problems? Check here first!

Start off by making sure that you're not using 2 software firewalls (just use 1) & double check the config of that firewall. XP's firewall will conflict with Norton's so turn the XP firewall off.
If you're using a modem/router, read the sections about UPnP & port forwarding within that sticky.

If you're stuck, post back with the answers to the questions in post #2 of the sticky