preload external movie - can anyone help?

Hi can anyone help me? I am new to actionscript 3 and I don't know how I should do the follow:

Basically what i want to do is

1. Have a flash swf (lets call this preloader.swf) that is 500px by 100px so quite a thin stage size, I want this movie to have a preloader animation and possible dynamic text displaying % loaded of an external file 'A.swf'.

2. When the external file A.swf is fully loaded and ready to play I want the 'preloader.swf' to be replaced by the loaded external swf which has a larger stage size 500px x 500px. I can see that this is like loading a movieclip in to the same level as the other but I do not know how to a) actionscript the first movie to know when the external file is totally loaded and b) to then trigger a loadmovie once the total frames is loaded.

Any help would be really appreciated.

Thanks

Terry