"floating" movieclip or fixed-positioned movielip

hi
I am developing a flash site at 2400x1600 pixels that is expected to show scrollbars (only few people with that big of a monitor). The challenge is to create a navigation bar (movieclip) that will always stay in the viewable part of the browser window and always 20px from the bottom edge, no matter how big the window is sized or where the user scrolls to.
It’s like one of those floating div’s in HTML that always stay put no matter where you scroll to. but this has to be done in flash.
can anyone give me pointers or show me how it’s done?
thanks!
pirco