Lottery Game Help

Hi, I am a first time poster on this, or any other board actually! But my problem is this:

I am trying to create a virtual lottery machine in Flash MX/8 and have hit upon a problem, how do I create a collision detection within a circle? I have managed to work with setting edges and haveing the balls bounce around with a given area that way, but doing it within a circle is proving tricky and I do not know where to start!

Here is an example of what I am trying to create:

[color=#008000]http://www.flashguru.co.uk/flash-experiment-bingo-machine/[/color]

unfortunately there is no tutorial for this and I have spent many an hour searching google and text books with not luck!

This is probably reasonably simple but any help will be very much appreciated!

Cheers

Ben