Use Firefox with the HTML validator addon to identify your validation errors.
On your page you have a closing div with no opening div (amongst other minor things): see screenshot,
This may be causing your error in ie, maybe not, but you have to fix what is definitely broken before looking for other reasons.
How to find things in files to correct your validation errors.
Remember that the page as you (and your browser) see it is made up of chunks of template files stuck together by php and with data inserted there by php so much of the html you see in the page source code (in the browser) does not exist written exactly the same in the zen cart files. So if you search for it, you wont find it.
This means that if you search (recommended is Windows GREP if you are developing on a local windows pc) for a line copied from the source code of your browser page like
<li><a href="http://www.mysite.co.uk/">Home</a></li>
You won't find it as the data bit in there – your site address – gets pulled out of the database or from a constants file at the moment the page is created (requested by your browser), it is not written in any page.
So, you should search for a nearby comment or id which generally does exist in the file somewhere exactly as it appears in the source.
eg:
<!--eof-optional categories tabs navigation display-->
You will find this near the end of
\includes\templates\template_default\common\tpl_header.php
AND
\includes\templates\YOUR_TEMPLATE_NAME\common\tpl_header.php
So maybe you have put an extra div at the end of the YOUR_TEMPLATE_NAME version
OR in the next bit that starts with
<div id="mainWrapper">
a search reveals this to be in
includes\templates\template_default\common\tpl_main_page.php
and maybe
includes\templates\YOUR_TEMPLATE_NAME\common\tpl_main_page.php
maybe you have put an extra div here?
Due to the nature of the beast (php) things are scattered around and you have to get used to searching for them, it does get easier..