CSS Problems: Advertise over 2 divs, layout problem

I have a div-layout on my asp.net page.

Left a div for menu, middle a div for content and right a div for a online user list. All divs are with float:left and height/width on his place and it works problemless.

Now I must have a advertise over the left and the middle div together.

My first try was to have it in the middle and set margin-left:-270px;. But the the advertise-div is OVER the menu an开发者_如何学God you cant click anything anymore.

My second try is to have it in the left div and overflow it easyly on the middle div, but that of course don't work, because the left menu div has a width: 300px; and exacly there end the banner.

here is it to see: http://s3.imgimg.de/uploads/4b247298bJPG.jpg

how to do?

It's hard to tell without seeing your HTML/CSS, but perhaps the easiest way would be to to put the advert in it's own <div> under all of the left, middle, right divs, and use something like margin-top: -110px to shift it up.

It's not a very clean solution. If you can't get this to work, or just plain don't like it, then post your code.