Accesing through Internet to multiple LAN machines using VNC

[GUACAMATO]

Hi everyone,

I have a Windows 2000 based LAN in my office with 5 Pcs connected to 
the LAN. I want to access any of the PCs through Internet using VNC to 
be able to see whats going on any of the PCs.

Being a DSL connection, I have opened in the router ports 5800, 5801, 
5900 and 5901 and redirected to each one of the LAN ips. This works 
with only one PC listening in the LAN. But if many PCs are listening 
then I try to connect, but I can't.

I have tried to change display numbers for each PC, but it doesn't work.

How can I configure the VNC in each PC in the LAN to be accessed from 
outside through Internet, one at a time?

Thanks in advance,
Quique.