つまりこうだろうか.
<html>
<head>
<title>Untitled Page</title>
<script type="text/javascript">
<!--
function setCursor(obj)
{
obj.style.position = "absolute";
var fn_onmousemove = document.documentElement.onmousemove;
document.documentElement.onmousemove = function(event)
{
if(fn_onmousemove != null)
fn_onmousemove.apply(this, arguments);
if(event == null) event = window.event;
obj.style.left = (event.clientX - obj.offsetWidth / 2) + "px";
obj.style.top = (event.clientY - obj.offsetHeight / 2) + "px";
};
}
function initialize()
{
setCursor(document.getElementById("cursor"));
//setCursor(document.getElementById("cursor2"));
}
//-->
</script>
</head>
<body onload="initialize();">
<div id="cursor">あ</div>
<!--<div id="cursor2">い</div>-->
</body>
</html>