codice:
<script language="javascript" type="text/javascript">
mouseX=0
mouseY=0
function el(name_ID) {
	return (document.all)?document.all[name_ID]:(document.getElementById)?document.getElementById(name_ID):null
}

function coords(e){
	if(!e) e = window.event
	mouseX = e.clientX+(window.opera?0:document.body.scrollLeft)
	mouseY = e.clientY+(window.opera?0:document.body.scrollTop)
}

function clicca(){
	with(el('mainBox').style) {
		left = mouseX+5+'px'
		top = mouseY+5+'px'
		visibility="visible"
	}
}

document.onmousemove = coords
</script>
...
<div id="mainBox" style="position:absolute;top:0px;left:0px;z-index:100;visibility:hidden">contenuto div nascosto</div>
...
<input type="button" onclick="clicca()" value="click me!" />
ciao