You also said "Wordpress" and "Widget" and that makes me thing you're working with the sidebar. Is it native theme code? If it is, it is horribly invalid. It nests the search form and heads inside li's:
<form action ="" id="search-widget">
<h3>My first widget</h3>
When the code is invalid, it most likely throws the pages into Quirks Mode which could be another aspect of what you're seeing. So the first step is to get it to validate [validator.w3.org] or as close as you can before tweaking the code for IE.
An easy quick-check: in FireFox, locate some background area, right-click and select View Page Info from the contextual menu. On the General tab you will see Render Mode.
I've found I have to really tweak the sidebar themes to get them to validate, but fortunately don't have to descend into the core code.