A dynamic IP changes every time you restart your computer.
(The IP from your router not your isp)
When you forward ports they are opened to one IP.
(for the explanation we will assume you forwarded ports to 192.168.2.3)
On a Dynamic IP, You forward your ports to 192.168.2.3
When you restart your pc the DHCP Lease(amount of time an IP is reserved)
hasn't run out so the router then tells your pc that it is 192.168.2.4
The problem with that is that the routers ports are still only open to 192.168.2.3 so by setting up a static ip you only have to configure ports once.
If you continue to consult comcast about this matter and they offer you a static-IP at additional cost it is not the same thing and you should gracefully decline.
A guide to setting up static ip has already been written and published on this site here:http://winmxworld.com/tutorials/static_ip.html
After that there are guides to forwarding ports for winmx by router here:http://www.portforward.com/english/applications/port_forwarding/WinMX/WinMXindex.htm
If you still cannot connect you may have a Firewall installed or the one built into windows turned on.
An explanation of them can also be found on this site with explanations of how to configure a few of them here:http://www.portforward.com/english/applications/port_forwarding/WinMX/WinMXindex.htm
If you need further assistance feel free to post a reply,
or alternately you can seek live help on winmx in this room: