I didn't see anything blatantly wrong with the html. You might try closing the input elements .
<input name="whatever" />
Otherwise, start from stratch and just gradually add elements until explorer stops working. Problems with css can often cause unexpected results.
BTW, I have been including the xhtml 1.1 dtd in my files for several years now. I have yet to come across a browser that misbehaves because of it. The browser may not understand all the xhtml but they all seem to render ok. And if you do use xhtml then you can submit your page to some of the validators out there which in turn will often point out syntax errors in your page.
