En explorer se usa event. Probá así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento sin título</title>
<script type="text/javascript">
function mover(e){
var evt=e || event, mouseX = Math.round (evt.clientX);
document.getElementById('test').innerHTML=mouseX;
}
onload=function(){
document.onmousemove=mover;
}
</script>
</head>
<body>
<div id="test"></div>
</body>
</html>