giusto per darti un esempio funzionante, copia e incolla

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="it" xml:lang="it">
<head>

<script type="text/javascript">
// <![CDATA[
 
function getEventMousePos(el, e) {
    var iScrollX = document.body.scrollLeft || document.documentElement.scrollLeft;
    var iScrollY = document.body.scrollTop || document.documentElement.scrollTop;

    e = e || window.event;
    if (e.currentTarget) {
        var pos = getElementPos(el);
        return {
            x : e.clientX - pos.x + iScrollX,
            y : e.clientY - pos.y + iScrollY
	    }

    }

    return {
		x : e.offsetX,
		y : e.offsetY
    }
}


function getElementPos(el) {
    var x = el.offsetLeft;
    var y = el.offsetTop;
    var parent = el.offsetParent;
    
    while (parent) {
	 x += parent.offsetLeft;
	 y += parent.offsetTop;
	 parent = parent.offsetParent;
    }

    return {
	 x : x,
	 y : y
    }
}


window.onload = function() {

    /* crei un riferimento al contenitore */
    var tuocontenitore = document.getElementById('cnt');
    /* all'evento 'mousemove' sul contenitore ricavi le coordinate */
    tuocontenitore.onmousemove = function(e) {
        var coords = getEventMousePos(tuocontenitore, e);
        document.getElementById('coordx').value = coords.x;
        document.getElementById('coordy').value = coords.y;
    }
}
// ]]>
</script>
</head>
    
<body>	
    <div style="width: 500px; height: 300px; background: #f4f4f9; position: relative;" id="cnt"> 
        x: <input type="text" id="coordx" />

        y: <input type="text" id="coordy" />
    </div>
</body>
</html>
Ciao