Actionscript Problem, any ideas?

I am creating a flash showcase of pupils work at my college and have the different showreels pop up in seperate movie clips using _root.attachMovie etc…I was wondering if anyone knows how I could get only one of these windows to be allowed on the screen at a time. I want to force the user to close the existing window before moving to the next window. The reason for this being that when you select an item from the menu when a window is already open it just opens on top or underneath of the existing window.

Any thoughts would be greatly appreciated. Thanx.