Humm..
This sounds like it could be your ISP, cant swear to that. If it is changing the ports WinMX uses may help with this.
Ports I suggest are 1720 TCP and 3339 UDP, these can be changed in WinMX by going to Settings > Internet Connections > Incoming TCP. .. change that to 1720.
Now go to IN/OUT UDP... change that one to 3339..
Now you will have to also change these in any routers, or firewalls that you have set, I dont believe I recall Norton needing specific ports so once the ports are set in XP firewall (if your using this) and any routers, you should be good to go after a restart of WinMX..
Let me know if this helps you.
Also since your a primary if your using the pie patch please be sure to use pg2 with our block list ..