<div id="fixed_menu" style="background:#000;position:fixed;width:100%;height:30px;left:0px;top:0px;"> </div>

<script type="text/javascript">   
$(function() {   
$(window).scroll(function(){   
var scrollTop = $(window).scrollTop();   
if(scrollTop != 0)   
$('#fixed_menu').stop().animate({'opacity':'0.2'},400);   
else   
$('#fixed_menu').stop().animate({'opacity':'1'},400);   
});   

$('#fixed_menu').hover(   
function (e) {   
var scrollTop = $(window).scrollTop();   
if(scrollTop != 0){   
$('#fixed_menu').stop().animate({'opacity':'1'},400);   
}   
},   
function (e) {   
var scrollTop = $(window).scrollTop();   
if(scrollTop != 0){   
$('#fixed_menu').stop().animate({'opacity':'0.2'},400);   
}   
}   
);   
});   
</script>