I was getting that same problem and found out it was the tpl_main_page file which you can find in zen cart/Includes/templates/templates_default/common - What i did was to open the file and change the http part at the bottom of that file to https and it cures the internet explorer 7 problem.
It was that simple for me and it's probably some image on your page needing this done to it.
My example below.
<a href="http://www.instantssl.com" id="comodoTL">SSL</a>
<script language="JavaScript" type="text/javascript">
COT("https://www.darkrealmfox.com/catalog/secure_site.gif", "SC2", "none");
</script>
I originally had the http part set to https. Also remember to set the link to https://www and not just https://