Using the VNC Repeater:
The VNC Repeater enables you to open up a single port on your firewall to the VNC Repeater machine (kind of a "gateway" or "proxy"), and use the VNC Viewer to remote control any local machines running the WinVNC Server. To follow are the instructions for this setup.
Download UltraVNC: http://uvnc.com
This "distribution" of the Open Source VNC project includes additional features including other compression algorithms for speed and flexibility, file transfer, chat, and MS Logon authentication, to name a few.
Install the VNC Server on a machine to be remote controlled
(WinVNC Server machine):
Select the necessary properties of WinVNC repeater