I've setup a SBS 2003 server, allowed web access, I can access it internally on the lan by using
http://<servername>/
I have also setup an A record for one of our domains to our companys external ip to see if I can use the web access, but this just asks me for the login for our router.
Our setup is a DrayTek Vigor2800 series router, the SBS server is behind this on our lan.