Gracias KarlanKas por responder. Aqui te paso el script para que le puedas echar un vistazo a ver si es alguna tonteria que por mis bajos conocimientos de javascript desconozco:
<!--
Nota: Las imágenes de este script se encuentran en la carpeta Exportar.
-->
<html>
<head>
<title> Carpetas IE </title>
<style>
<!--
#foldheader{cursor:hand ; font-weight:bold ;
list-style-image:url(fold.gif)}
#foldinglist{list-style-image:url(list.gif)}
//-->
</style>
<script language="JavaScript1.2">
<!--
/**
* Based on Folding Menu Tree
* Dynamic Drive (
www.dynamicdrive.com)
*/
var head="display:''";
img1=new Image();
img1.src="fold.gif";
img2=new Image();
img2.src="open.gif";
function change(){
if(!document.all)
return
if (event.srcElement.id=="foldheader") {
var srcIndex = event.srcElement.sourceIndex
var nested = document.all[srcIndex+1]
if (nested.style.display=="none") {
nested.style.display=''
event.srcElement.style.listStyleImage="url(pc.gif) "
}
else {
nested.style.display="none"
event.srcElement.style.listStyleImage="url(pc.gif) "
}
}
}
document.onclick=change
//-->
</script>
</head>
<body bgcolor="#FFFFFF">
<ul>
<li id="foldheader">Laboratorio</li>
<ul id="foldinglist" style="display:none" style=&{head};
<li><a href="http://www.cnn.com">PCINF159</a></li>
<li><a href="http://www.abcnews.com">PCINF160</a></li>
<li><a href="http://www.vancouversun.com">PCINF162</a></li>
</ul>
<li id="foldheader">Informatica</li>
<ul id="foldinglist" style="display:none" style=&{head};>
<li><a href="http://www.gamespot.com">PCINF101</a></li>
<li><a href="http://www.happypuppy.com">PCINF102</a></li>
<li><a href="http://www.gamecenter.com">PCINF103</a></li>
</ul>
<li id="foldheader">Telematica</li>
<ul id="foldinglist" style="display:none" style=&{head};>
<li><a href="http://www.download.com">PCINF201</a></li>
<li><a href="http://www.hotfiles.com">PCINF202</a></li>
<li id="foldheader">PCINF203</li>
<ul id="foldinglist" style="display:none" style=&{head};>
<li><a href="http://www.windows95.com">tarjeta red 1</a></li>
<li><a href="http://www.shareware.com">tarjeta red 2</a></li>
</ul>
<li><a href="http://www.epsig.inforg.uniovi.es">Industriales</a></li>
<li><a href="http://www.shareware.com">Telecomunicaciones</a></li>
</ul>
</ul>
<script language="JavaScript1.2">
<!--
/**
* Get cookie routine by Shelley Powers
* (
[email protected])
*/
function get_cookie(Name) {
var search = Name + "="
var returnvalue = "";
if (document.cookie.length > 0) {
offset = document.cookie.indexOf(search)
// if cookie exists
if (offset != -1) {
offset += search.length
// set index of beginning of value
end = document.cookie.indexOf(";", offset);
// set index of end of cookie value
if (end == -1) end = document.cookie.length;
returnvalue=unescape(document.cookie.substring(off set, end))
}
}
return returnvalue;
}
if (get_cookie(window.location.pathname) != ''){
var openresults=get_cookie(window.location.pathname).s plit(" ")
for (i=0 ; i < openresults.length ; i++){
foldinglist[openresults[i].style.display=''
document.all[foldinglist[openresults[i].sourceIndex -
1].style.listStyleImage="url(open.gif)"
}
}
if (document.all){
var nodelength=foldinglist.length-1
var nodes=new Array(nodelength)
var openones=''
}
function check(){
for (i=0 ; i <= nodelength ; i++){
if (foldinglist[i].style.display=='')
openones=openones + " " + i
}
document.cookie=window.location.pathname+"="+openo nes
}
if (document.all)
document.body.onunload=check
//-->
</script>
</body>
</html>
menus tipo arbol con javascript 
!
