<!DOCTYPE html>
<html>
<head>
<style type="text/css">
html, body{height:1000px;}
.block{width:300px;height:300px;background:#000000;position:fixed;}
</style>
<script src="http://code.jquery.com/jquery-1.8.2.min.js"></script>
<script type="text/javascript">
$(window).scroll(function(){
var scrolls=$.browser.msie?document.body.scrollTop:$(this).scrollTop();
if(0<scrolls) {
$('.block').css('opacity','0.5');
} else {
$('.block').css('opacity','1');
}
});
</script>
</head>
<body>
<div class="block"></div>
</body>
</html>