Your navtab template is all messed up. Your html is totally incorrect. firefox and other browsers are most likely "correcting" it for you and IE is giving you exactly the mess you are outputting. This is the html you have for what should be the third tab (you have a blank <li> </li> for the second tab, and your fourth tab, according to your html, is the About Us tab).
HTML Code:
<li class="selected" style="padding:0px">
<ul class="floatcontainer">
<li><a rel="help" href="faq.php" accesskey="5">FAQ</a></li>
<ul class="popupbody popuphover">
<li>
<a href="forumdisplay.php?do=markread&markreadhash=guest" onclick="return confirm('Are you sure you want to mark all forums as being read? This cannot be undone.')">Mark Forums Read</a>
</li>
</ul>
</li>
<ul class="popupbody popuphover">
<li><a href="showgroups.php" rel="nofollow">
Fellows & Board members
</a></li>
</ul>
</li>
</ul>
</li>
I suggest you redo your navbar template and correct whatever is supposed to show up here.
If you need more help, you really need to post int he main forums as this isn't a thread for fixing html problems in your navbar template.