Web design question

Hi–I’m trying to put together my first flash site. The problem I’m having is this:

I have my background and navigation on frame 1 on seperate layers. I’d like each button in the navigation to load an external .swf in a consistent and particular area of the movie. For example the main movie is 800x600 and the ext.swfs are 400x300. How can make these load centered in the main movie?
Any suggestions?

Thanks. Sean.