On the computer that will be sharing the desktop, go to System > Preferences > Remote Desktop
If you want others to just see your desktop, but not be able to make changes, enable Allow other users to view your desktop only. If they should be able to change settings (e.g. repair your system if there are problems), enable Allow other users to control your desktop as well.
If someone connects to your desktop and you want to be prompted to block or allow that connection, enable Ask you for confirmation. This makes sense only if someone is actually sitting in front of the system. If you want to connect to your office desktop or any other system that only you have access to, then don’t enable this option
However, with Ask you for confirmation off, what you certainly want to do is set a password for your remote desktop (without a password anyone who happens to find out your system’s address – e.g. by scanning the network – can access your desktop)
If you’re behind a router or firewall, you may need to allow or forward port 5900 to that computer.
Sourced from http://www.howtoforge.com/configure-remote-access-to-your-ubuntu-desktop March 28th 2010