The page displays OK in Firefox, but not in MSIE.

This leads me to think that the following is the problem...

Open this file for editing:

includes/templates/template_default/common/tpl_header.php

and find the following two lines (near line 60 - 65)

HTML Code:
<div class="clearBoth"></div>
  <!--eof-navigation display-->
COMMENT out the div clear both line - like this...

HTML Code:
<!--  EDIT BEGIN  <div class="clearBoth"></div> EDIT END -->
  <!--eof-navigation display-->
Save the edited file to:

includes/templates/yeom/common/

NB: if you already have an override of this file, edit the override and put it back in the proper yeom folder.