The problem isn't that you can't access the domain, it's that you can't access your external IP address from behind your router. Due to the way NAT works, you can't make a connection from, say, 192.168.1.101 to the external IP which is then forwarded to the internal IP of your server. It simply can't allow both the inbound and outbound connections to the same machines.
The way to get around it is to make a HOSTS file entry. In WinXP/2k (forget where in Win9x), in the windows\system32\drivers\etc folder is a file named HOSTS, which is just a text file. Open it in Notepad and add a line like this:
<serverIP> <domainname>
You'll see examples to help you. This will override the DNS entry, so that your browser will connect directly to the server rather than trying to send the traffic to the external IP.