you will create something like the above animation.

Creating Random Motion in Flash MX:

Create a new movie in Flash MX. Set the width and height to anything you want.

Now, draw A Square and make it a movie clip. To convert something into a movie clip, select it and press F8. From the Convert to Symbol Dialog box that appears, select the Movie Clip option and press OK:

Once you have created your movie clip, here comes the complicated process of Copying and Pasting. Right click on Frame 1 (can be a keyframe or a blank keyframe) of your movie and select Actions.

Copy and paste the following code into your Actions dialog box:

After the rigorous process of Copying and Pasting you undertook in Step
3, right click on your movie clip and select Actions. From the Actions
dialog box that appears, copy and paste the following code:

Now, copy and paste your movie clip as many times as you want
on your timeline to create multiple, randomly-moving objects.
You may even create a separate movie clip with a different object in it.
Just make sure the above 3 lines of code are added to each movie clip that you
want to move randomly:

NOTE:

The above code is set to 300x200x so you will see it is easy to change just change the width
and height to what you want. which is in the first part of the actionscript.

Click here to download the source file to see how I have done it

IMPORTANT

I have to give full credit to http://www.kirupa.com for this tutorial and superbreener who is a member of the kirupaforum.com forum for the code