I just turned my screen resolution right down and can see your problem.
How annoying for you.....
I have signed over the banner management to an advertising company and the most effective banner placement (read $$$
) is this leaderboard header size.
When I have got some more time I will look at perhaps swapping the menus on the left hand side over which should resolve the problem or maybe just moving the banner down below the header.
I should really make sure that the forum posts are viewable at a minimum of 800x600 although less than 5% of the visitors have it set that low.