Allowing external Connections

I am running a linux Machine that acts as my internet gateway.
I also have a windows 2000 Server connected to a another network through a secured connection I have been given an IP to connect to. I would like to allow an IP outside of my network through the internet to connect to the IP on the Secured connection.

not sure how u need to do it, but it soulds to me like VPN is what u are wanting.