You will need to have an MTA (Message Transfer Agent) of some kind on the Windows Server as well as all pertinent ports open and forwarded to the server. A reverse DNS record would also be very helpful. Windows Server 2003 does have an SMTP engine that you can install on it, but be careful or you'll be pwnz0rd 6 ways from Sunday in 2 seconds flat.
Check out this comparison of mail servers, find one for Windows (Kerio is super and really cheap) and knock yourself out with the documentation. Hang out on the forums for that mail server program, read, ask and learn. That's the best way to learn. To quote Nike, "Just do it"