desktop sharing server question.

[hilz]

Hi all,
I am looking for an open source server that i can run on ubuntu that 
provides services similar to gotomeeting or webex.
I am aware of vino, but that is not what i want.

What I want is to run this server on ubuntu (exposed to the internet), 
and have two windows machines (running behind a firewall with NAT) 
connect to this ubuntu server and be able to share their desktops with 
each other.

is there such a thing?
thanks