function handleEvent(e)
{
	if (!e) var e = window.event;
	var posx = 0;
	var posy = 0;
	if (!e) var e = window.event;
	if (e.pageX || e.pageY)
	{
		posx = e.pageX;
		posy = e.pageY;
	}
	else if (e.clientX || e.clientY)
	{
		posx = e.clientX + document.body.scrollLeft;
		posy = e.clientY + document.body.scrollTop;
	}
	// posx and posy contain
	// the mouse position relative to the document
	// Do something with this information
}

http://evolt.org/article/Mission_Impossible_mouse_position/17/23335/index.html

Snagged from pp]{
It's relative to document, not the visible area in the window.

Get mouse coordinates relative to document

JavaScript