This will work in IE, I am not sure about the other browsers since I can't test them at work.
HTML Code:
<html>
<head>
<title>Mouse Positions</title>
<script type = "text/javascript">
function Show_Coords(){
frmMain.txtX.value = event.clientX;
frmMain.txtY.value = event.clientY;
}
document.onmousemove = Show_Coords;
</script>
</head>
<body>
<form ID = "frmMain">
<label>x:</label><input type = "text" value = "0" ID = "txtX" />
<label>y:</label><input type = "text" value = "0" ID = "txtY" />
</form>
</body>
</html>