Try looking in the tempalte...

overall_header.tpl and
overall_footer.tpl

There at tags at the start of the html in
overall_header, then closing tags in
overall_footer.tpl

the amount of open then closed tags need to match. In some cases, you actually need another set of maybe
</td>
</table>
tags in the overall_footer.
Jut play around with it, you won't hurt much messing with those files.

mikem