Content area as different movie clips

Hi there, I’m sure this is a pretty simple question for some of you.

I’m building a site in Flash, and at the moment, I have my buttons, and a few other bits and pieces.

When the user navigates via the buttons, I want just one small part fo the stage to change, and everything else to remain.

There is an example of this here

As you can see, when the user clicks on a different button, only a small portion of the right side loads, with a pretty neat transition.

I’d like to know how this can be achieved.

Thanks in advance