#dark {
	background-color: black;
	top: 0;
	left: 0;
	position: fixed;
	width: 100%;
	height:100%;
	z-index: 99;
	background-repeat: no-repeat;
	background-position: center center;
}

<!--[if lte IE 7]> 
#dark { 
position: absolute; 

top: expression(eval(document.body.scrollTop)); 
width: expression(eval(document.body.clientWidth)); 
} 
<![endif]-->