Off Stage Flash (menu)

If I have a web page that has a header with a width and height of 800px x 100px and I want that to be a menu, bu then to have the dropdown part of the menu go over the html, how do I do that? Is my stage 800 x 100? I guess I’m not sure where to start with this.

BMW has it on their site here: http://www.bmwusa.com/vehicles/3/328isedan/default

I know some of their pages are all flash, but there are a few that are html with a flash menusystem that drops down over the html.

Any ideas?