Thanks vwgolfman, but using/creating a sitemap and securitytrimming is not the problem. The problem is building a horizontal menu (after a few hours of googleing I came to the conclusion that a tab menu doesn't seem possible. I'm going with a horizontal menu now).
What I would like to have is a horizontal menu (using a sitemap as input) that displays the highest level in my sitemap. When I mouseover a menu item I would like to see the second menu (scrolling down from the menu item or something) associated with the mouseovered upperlevel item.