Proxys and remote connections

I have recently set up a small network for a friend of mine with an ADSL internet connection. I have installed Analogue X proxy to enable other computers on the internal network access to the ADSL link. These computers all point to the proxy for external access. I am using WinXP Pro.

How can I allow access to one of the internal computers from an external computer with this setup?

I am very green with firewalls and proxys and I have been reading as much on the net as I can get my hands on about these. Unfortunately no one seems to have raised my specific situation. So I am.

Can anyone help me??

How can I allow access to one of the internal computers from an external computer with this setup?

Through AnalogX you have access to FTP, HTTP, NNTP serveses.
That means internal computers should have FTP, HTTP, NNTP servers installed.