Changing connection address for server

[Rudolf Ladyzhenskii]

Hi, all

I have following problem:

PC is connected to the Internet via cheap USB ADSL modem.
Authentication is happening on PC side, I beleive (this is a machine I
am trying to connect to and I do not have much technical info on it).
ipconfig on that machine produces 2 IP addresses:
172.22.xxx.xxx as "Ethernet connection" and a public IP on ppp connection.

I beleive that 172.xxxx is an IP address that modem assigns to a
Windows box, but when connection is up, ppp gets real public IP.

Anyway, VNC server always listens on the 172.xxxx IP address and I can
not connect to it from outside. Is there a way to force it to use
whatever address ppp connection gives it?
I understand that it will be difficult for VNC server to monitor PPP
conenction and I am happy to start the server manually after
connection is up.

Any thoughts?

Thanks,
Rudolf
P.S. My uncle half-way across the world got himself a computer and I
am trying to help him learn how to use it.