First of all you need to know that twenty eleven theme is in HTML5 and IE6, IE7 doesn’t supports HTML5 (http://www.quirksmode.org/dom/html5.html)
Secondly if you are having a menu problem in IE 7, then there is a fix available which I found on some website
#branding #searchform {
display: none;
position: absolute;
right: 7.6%;
text-align: right;
top: 3.8em;
}