Ho risolto così:

codice:
var offsetxpoint=-60 //Customize x offset of tooltip
var offsetypoint=20 //Customize y offset of tooltip
//le prossime due righe sono state commentate da tombo
//var ie=document.all
//var ns6=document.getElementById && !document.all
var enabletip=false
//commentato il contenuto dell'if ed esplicitato il valore
//if (ie||ns6)
if (document.all||document.getElementById && !document.all)
var tipobj=document.all? document.all["dhtmltooltip"] : document.getElementById? document.getElementById("dhtmltooltip") : ""

function ietruebody(){
return (document.compatMode && document.compatMode!="BackCompat")? document.documentElement : document.body
}

function ddrivetip(thetext, thecolor, thewidth){
	if (ns6||ie){
	if (typeof thewidth!="undefined") tipobj.style.width=thewidth+"px"
	if (typeof thecolor!="undefined" && thecolor!="") tipobj.style.backgroundColor=thecolor
	tipobj.innerHTML=thetext
	enabletip=true
	return false
	}
}

function positiontip(e){
	if (enabletip){
//commentate in favore delle due if successive (infatti anche se formalmente corrette le prossime due righe vengono ignorate da ie) 
	//var curX=(ns6) ? e.pageX : event.x+ietruebody().scrollLeft;
	//var curY=(ns6) ? e.pageY : event.y+ietruebody().scrollTop;
		if (document.getElementById && !document.all) {
		var curX = e.pageX;
		var curY = e.pageY;
		}
		if (document.all) {
		var curX = event.x+ietruebody().scrollLeft;
		var curY = event.y+ietruebody().scrollTop;
		}
//Find out how close the mouse is to the corner of the window
	var rightedge=ie&&!window.opera? ietruebody().clientWidth-event.clientX-offsetxpoint : window.innerWidth-e.clientX-offsetxpoint-20
	var bottomedge=ie&&!window.opera? ietruebody().clientHeight-event.clientY-offsetypoint : window.innerHeight-e.clientY-offsetypoint-20

	var leftedge=(offsetxpoint<0)? offsetxpoint*(-1) : -1000

//if the horizontal distance isn't enough to accomodate the width of the context menu
	if (rightedge<tipobj.offsetWidth)
//move the horizontal position of the menu to the left by it's width
	tipobj.style.left=ie? ietruebody().scrollLeft+event.clientX-tipobj.offsetWidth+"px" : window.pageXOffset+e.clientX-tipobj.offsetWidth+"px"
	else if (curX<leftedge)
	tipobj.style.left="5px"
	else
//position the horizontal position of the menu where the mouse is positioned
	tipobj.style.left=curX+offsetxpoint+"px"

//same concept with the vertical position
	if (bottomedge<tipobj.offsetHeight)
	tipobj.style.top=ie? ietruebody().scrollTop+event.clientY-tipobj.offsetHeight-offsetypoint+"px" : window.pageYOffset+e.clientY-tipobj.offsetHeight-offsetypoint+"px"
	else
	tipobj.style.top=curY+offsetypoint+"px"
	tipobj.style.visibility="visible"
	}
}

function hideddrivetip(){
	if (ns6||ie){
	enabletip=false
	tipobj.style.visibility="hidden"
	tipobj.style.left="-1000px"
	tipobj.style.backgroundColor=''
	tipobj.style.width=''
	}
}

document.onmousemove=positiontip
Grazie a tutti per l'interessamento.
Ora funziona senza errori con tutti i browsers compreso IE.