If your ISP is currently hosting your email, my first question would be whether you have a registered domain for the company? This will be very important to determining the next steps.
If you already have a domain set up, you will need to see about changing your MX records to point that to your IP address (your web host would do this for you). Once that is done, you should be able to use the Internet and Email Wizard on the server, setting it up with your existing domain name. Make sure your firewall is set to allow traffic through ports 25 and 110, and you should be good.
If you do not have an existing domain set up, that will be where you need to start. You could still use the Pop3 connector, but it sounds as if you want the total package. If you are not comfortable, it might be worthwhile to bring in a contractor to do the initial set up … but if you are wanting to bone up and learn to do more of the functions yourself, the SBS 2003 Administrator’s Companion is a great resource for many of the functions and situations you find.