your top level menu does not have a width specified.
for compatibility with IE, you must specify a width and height for the menu items using a pixel measurement.
leaving the width or height blank, or using auto will not work in IE.
wher you are trying to give a unique width to each top level item, you will need to add an inline width to the menu html
for example:
<li><span><a href="index.html" style="width: 80px;">HOME</a></span></li>