function CursorPosition(e) 
{
	if (!e) e = window.event;
	
    var cursor = {x:0, y:0};
	
	if (!e)
	{
		return cursor;
	}
	
    if (e.pageX || e.pageY) {
        cursor.x = e.pageX;
        cursor.y = e.pageY;
    } 
    else {
        cursor.x = e.clientX + 
            (document.documentElement.scrollLeft || 
            document.body.scrollLeft) - 
            document.documentElement.clientLeft;
        cursor.y = e.clientY + 
            (document.documentElement.scrollTop || 
            document.body.scrollTop) - 
            document.documentElement.clientTop;
    }
    return cursor;
}

// Then you could do
function popup(e)
{

   var pos = CursorPosition(e);
   var popElem = document.getElementById('popup');
   popElem.style.display = "block";
   popElem.style.left = pos.x + '32';
   popElem.style.top = pos.y + '32';
}