There are a number of options here for you, but it could be worth running Mail Enable (http://www.mailenable.com/)
Its free and is a pretty good product.
Simply what you need to do is configure your Domains DNS MX Record to change from its current ip to the IP of your Broadband/Cable Router (providing its a Static IP You have).
Setup port forwarding to your Windows 2003 Server hosting Mailenable, port 25.
There is a sneaky way to get this to work if you have a dynamic ip.
Register a free noip domain at http://www.no-ip.com and run the no-ip client on your server to update your ip when it changes.
What you need to do is change your MX Record to point to the Address that you select (e.g mydynamicip.redirectme.org).
For example i run this on my exchange server at home my mx record for rallycross.ie points to oracleexch.redirectme.net and my router is configured to pass all port 25 comms to ghost (my exchange server).
This should get you up and running