It's tough to testing things like this out in old browsers that don't have in inspector so you can't really see the elements.
Since there are a few elements overlapping in the header, I'd suggest giving each a different background color so you can see how they are all placed in IE8, the slideshow might just be hidden behind something. This should be pretty easy to spot if the nav is red and the logo is green.
There are a few things that I spotted that might be causing issues.
One is this element that is being added after the header:
background-position: center center;
content: " ";
I'm not sure what that is even for, but I can see how it might not layout properly in IE8.
Also, there is a clear after the header:
<div class="cleared reset-box"></div>
But it has a table display, try removing this and seeing what happens: