#topmenu{position:absolute;top:100px;background:#4b4a4a;width:100%;margin:0 auto;clear:both;overflow:hidden;z-index:998;left:0}

<script type="text/javascript">
jQuery(document).ready(function() {
var topmenu = jQuery("#topmenu");
var topmenu_top = topmenu.offset().top;
reset_topmenu_top(topmenu, topmenu_top);
jQuery(window).scroll(function() {
reset_topmenu_top(topmenu, topmenu_top);
});
});
function reset_topmenu_top(topmenu, topmenu_top) {
var document_scroll_top = jQuery(document).scrollTop();
if (document_scroll_top > topmenu_top) {
topmenu.css('top', document_scroll_top);
}
if (document_scroll_top <= topmenu_top) {
topmenu.css('top', topmenu_top);
}
}
</script>