Windows already has a remote access system that you can use and connect using
vinagre application available for Ubuntu.
Right click on My Computer > Properties > Remote tab
You just need to configure it and ask the network administrator to redirect the correct ports to you Windows computer.
The necessary port is TCP 3389, redirect connections on this port to your TCP 3389 port in your computer and you will be able to use the
vinagre (installed by default on Ubuntu 11.10) to remotely connect.
If you do not want to install any extra software quicksharescreen.com should allow you to support a customer over a web based interface that run with Java.
Its still in beta fase but it should work.
Has an alternative you can use others, here is a list of sites that allow you to share a desktop over the web and are cross platform: