Access Ubuntu PC behind NAT router by software service

How to access Ubuntu PC behind NAT router without reconfiguring NAT router, but by pure software (apps or services)?

Here is the problem description:

1. In my office I have Ubuntu PC which has VNC enabled. Ubuntu PC has private IP address and is behind not just one but two Internet routers, so configuring NAT on them is not possible (I do not have access to them).
2. At my home, I have my own LAN workgroup and one server PC (windows xp) which is directly connected to Internet and has public ip address.

So what I want is a way to access VNC server on my Ubuntu PC directly from PC which is on LAN at my home. As you can see both computers have private IP addresses and both are behind NAT routers. I would need something like LogMeIn except not for Windows but for Ubuntu.

Also I thought maybe to write program in C++ or Java which should consist of servers and clients which could do port forwarding and I have some ideas but I would really like to see if there is already a way to do it in Ubuntu without reinventing a wheel.

You should look into TeamViewer for Linux. According to the FAQ it supports this.