Main Categories in Header, Subcategories in Sideboxes

I am trying to create a site where you can have Main Categories always shown in a top header menu. Then with each category you choose, you will have only the subcategories (of that Main Category you choose) show in a side menu.

I can't find anything in contributions and am having a hard time determining if this can be done with templates and overrides.

Here's an example site that runs zen cart and does something similar to what I'm looking for: http://www.drogistenapotheek.nl/shop/

If you hover your mouse over the top menu "HOOFDMENU", it shows a lot of subcategories you can click on. Then that Subcategory is shown in a left hand side box.

Please help. Thanks!