Ho trovato lo script che fa a caso nostro, ti commento nel codice le parti che non capisco:

codice:
<HTML>

<HEAD>
<TITLE>HTML.it - il sito italiano sul Web publishing | Un JavaScript a settimana | HTML.it</TITLE>

<style>
A:link, A:visited { text-decoration: none }
A:hover { text-decoration: underline }
</style>

<style type="text/css">
#divMenu0{position:absolute; top:0; left:30; visibility:hidden; font-family:verdana,helvetica; font-size:15px; font-weight:bold}
#divMenu1{position:absolute; top:0; left:150; visibility:hidden; font-family:verdana,helvetica; font-size:15px; font-weight:bold}
#divMenu2{position:absolute; top:0; left:350; visibility:hidden; font-family:verdana,helvetica; font-size:15px; font-weight:bold}
</style>
<script language="JavaScript" type="text/javascript">


function lib_bwcheck(){
	this.ver=navigator.appVersion
	this.agent=navigator.userAgent
	this.dom=document.getElementById?1:0
	this.opera5=this.agent.indexOf("Opera 5")>-1
	this.ie5=(this.ver.indexOf("MSIE 5")>-1 && this.dom && !this.opera5)?1:0;
	this.ie6=(this.ver.indexOf("MSIE 6")>-1 && this.dom && !this.opera5)?1:0;
	this.ie4=(document.all && !this.dom && !this.opera5)?1:0;
	this.ie=this.ie4||this.ie5||this.ie6
	this.mac=this.agent.indexOf("Mac")>-1
	this.ns6=(this.dom && parseInt(this.ver) >= 5) ?1:0;
	this.ns4=(document.layers && !this.dom)?1:0;
	this.bw=(this.ie6 || this.ie5 || this.ie4 || this.ns4 || this.ns6 || this.opera5)
	return this
}
var bw=new lib_bwcheck()


var tMove=10;


var tSpeed=40


var tMoveOnScroll=true


var tShow=20


function makeMenu(obj,nest,show,move,speed){
    nest=(!nest) ? "":'document.'+nest+'.'
	this.el=bw.dom?document.getElementById(obj):bw.ie4?document.all[obj]:bw.ns4?eval(nest+'document.'+obj):0;
  	this.css=bw.dom?document.getElementById(obj).style:bw.ie4?document.all[obj].style:bw.ns4?eval(nest+'document.'+obj):0;
	this.x=this.css.left||this.css.pixelLeft||this.el.offsetLeft||0
	this.y=this.css.top||this.css.pixelTop||this.el.offsetTop||0
	this.state=1; this.go=0; this.mup=b_mup; this.show=show; this.mdown=b_mdown;
	this.height=bw.ns4?this.css.document.height:this.el.offsetHeight
	this.moveIt=b_moveIt; this.move=move; this.speed=speed



/* parte che non capisco */
    this.obj = obj + "Object"; 	eval(this.obj + "=this")
/* parte che non capisco */



}
function b_moveIt(x,y){this.x=x; this.y=y; this.css.left=this.x; this.css.top=this.y}

function b_mup(){
	if(this.y>-this.height+this.show){
		this.go=1; this.moveIt(this.x,this.y-this.move)
		setTimeout(this.obj+".mup()",this.speed)
	}else{this.go=0; this.state=1}
}

function b_mdown(){
	if(this.y<eval(scrolled)){
		this.go=1; this.moveIt(this.x,this.y+this.move)



/* parte che non capisco */
		setTimeout(this.obj+".mdown()",this.speed)
/* parte che non capisco */




	}else{this.go=0; this.state=0}
}

function moveTopMenu(num){
	if(!oMenu[num].go){
		if(!oMenu[num].state)oMenu[num].mup()
		else oMenu[num].mdown()
	}
	for(i=0;i<oMenu.length;i++){
		if(i!=num && !oMenu[i].state){ oMenu[i].mup()}
	}
}

function checkScrolled(){
	for(i=0;i<oMenu.length;i++){
		if(!oMenu[i].go){
			y=!oMenu[i].state?eval(scrolled):eval(scrolled)-oMenu[i].height+oMenu[i].show
			oMenu[i].moveIt(oMenu[i].x,y)
		}
	}
	if(bw.ns4||bw.ns6) setTimeout('checkScrolled()',40)
}

function topMenuInit(){
	oMenu=new Array()
	oMenu[0]=new makeMenu('divMenu0',"",20,10,20)
	oMenu[1]=new makeMenu('divMenu1',"",tShow,tMove,tSpeed)
	oMenu[2]=new makeMenu('divMenu2',"",20,10,20)
	scrolled=bw.ns4||bw.ns6?"window.pageYOffset":"document.body.scrollTop"

	for(i=0;i<oMenu.length;i++){
		oMenu[i].moveIt(oMenu[i].x,-oMenu[i].height+oMenu[i].show)
		oMenu[i].css.visibility='visible'
	}
	if(tMoveOnScroll) bw.ns4||bw.ns6?checkScrolled():window.onscroll=checkScrolled;
}


onload=topMenuInit;
</script>

</HEAD>
<BODY  marginleft="0" marginheight="0" BGCOLOR="white" vlink="blue" link="blue">



<div id="divMenu0">
	Link1


	Link2


	Link3


	Link4


	MENU 1
</div>
<div id="divMenu1">
	[img]01.gif[/img]

	[img]02.gif[/img]

	MENU 2
</div>
<div id="divMenu2">
	Prova

	Prova con scritta senza link



	MENU 3
</div>
























</div>

<font size=1 face="arial">
<div align="center">
<SCRIPT LANGUAGE="JAVASCRIPT">
<!--
var browName = navigator.appName;
var browDateTime = (new Date()).getTime();
var browVersion = parseInt(navigator.appVersion);
var ua=navigator.userAgent.toLowerCase();
var adcode='';
if (browName=='Netscape'){
     if (browVersion>=5)
          { document.write('<ifr'+'ame src="http://ad.html.it/banman.asp?ZoneID=36&Task=Get&Browser=NETSCAPE6&X=' + browDateTime + '" width=468 height=60 Marginwidth=0 Marginheight=0 Hspace=0 Vspace=0 Frameborder=0 Scrolling=No></ifr'+'ame>'); }
     else if ((browVersion>=4)&&(ua.indexOf("mac")==-1))
          { document.write('<S'+'CRIPT src="http://ad.html.it/banman.asp?ZoneID=36&Task=Get&Browser=NETSCAPE4">');
          document.write('</'+'scr'+'ipt>');
          document.write(adcode); }
     else if (browVersion>=3)
          { document.write(''); } }
if (browName=='Microsoft Internet Explorer')
     { document.write('<ifr'+'ame src="http://ad.html.it/banman.asp?ZoneID=36&Task=Get&X=' + browDateTime + '" width=468 height=60 Marginwidth=0 Marginheight=0 Hspace=0 Vspace=0 Frameborder=0 Scrolling=No></ifr'+'ame>'); }
// -->
</script>
<noscript>
    <a href="http://ad.html.it/banman.asp?ZoneID=36&Task=Click&Mode=HTML&PageID=7414" target="_new">
    </a>
</noscript>
</div>

<div id="divDescription">
 </div>

</font></BODY>
</HTML>