Вот так:
<script language="Javascript">
var ca,var in_l;ca=true;
in_l=setInterval(pos_l,20);
function pos_l(){
ar.width=document.body.clientWidth+document.body.scrollLeft;
ar.top=document.body.scrollTop;
}
</script>
<body>
<DIV name=ar id=ar style="position:absolute; top:0; left:0; visibility:visible; background-color:white">
Твой слой</DIV>