We use an application called "ShowMyPC". The writers of the software are available at www.showmypc.com. It is a free for use application that allows 1 computer to remotely control the other.
The application does not install anything on your computer. It is a one time use app, so once you close the program, we can no longer remotely control your computer.
Click here for the Windows Client
Click here for the Mac Client (beta)