It was not clear to me if the PC was external to your network or internal and just accessing the external IP. If the PC is external to your network, then I'm not sure why one client can access your server and another cannot. Thus, I'm assuming the PC is internal accessing the external IP. If internal, the following may be useful.
I have a router behind my DSL modem which is also a router. There are 3 steps to share a Windows machine outside my network.
1) open port on Windows firewall
2) port forward on router
3) port forward on DSL modem or switch modem router to bridge mode
Step 1 will allow another computer to access your windows server directly from the network by specifying the IP of that machine. Step 2 allows you to access the windows machine with the LAN address the router and have it forward to the PC. Step 3 will allow you to access the PC from outside the network by calling the external IP which will then forward to the router which will then forward to the PC. Without step 3 you can't get to the system from outside your network but, depending on how your modem handles it, you may be able to connect to your external IP from inside the network. There is a setting called "loopback" that may be enabled on your modem that will allow this.