C# Proxy Relay

I am looking for a creative problem solver who can create some sort of proxy service / ssl tunnel on a windows machine. Basically, in order to access our order entry system, we have to connect through our office IP. So we'd like to run a process on a windows pc that is located at our office that allows an outside sales rep to connect to this machine as you would any other proxy server. The biggest problem here is our fire wall. We can't login to the router to create port exceptions or direct the packets, so the program would have to create an outbound connection to an external server that listens for incoming connections. I'm not sure exactly how this would work, but am sure it's possible. We'd prefer the program be written in C#. Here's an example of how I see this *might* work: Computer B has the C# program installed. When ran, the program creates an outbound connection to external server C and listens for connections. Computer A sets a proxy server in internet explorer, which is the ip of external server C - and maybe the port specified correlates to Computer B. The connection is established and Computer A is able to browser the internet as Computer B.