Ho sbagliato a trascrivere, il secondo script richiama style.js
Solo lo slide show utilizza la funzione onload, posto il codice così forse qualcuno potrebbe capire dove ho sbagliato

per comodità ho evidenziato in blu tutto il codice che serve al funzionamento del menù, in rosso quello relativo allo slide show

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="it">

<head>
<title></title>
<meta http-equiv="Content-Language" content="it">
<meta http-equiv="Content-type" content="text/html; charset=iso-8859-1" />

<style>
<!--
 p.MsoNormal
	{mso-style-parent:"";
	margin-bottom:.0001pt;
	font-size:12.0pt;
	font-family:"Times New Roman";
	margin-left:0cm; margin-right:0cm; margin-top:0cm}
-->
</style>

<script type="text/javascript" language="javascript" src="sniffer.js">

/***********************************************
* Top Navigational Bar III v3.5 (By BrotherCake- brothercake.com)
* Exclusive permission by Dynamicdrive.com to include script in archive
* For this and 100's more DHTML scripts, visit http://www.dynamicdrive.com/
* This notice MUST stay intact for legal use
***********************************************/

</script>
<script type="text/javascript" language="javascript1.2" src="custom.js"></script>
<script type="text/javascript" language="javascript1.2" src="style.js"></script> 

<script type="text/javascript" language="JavaScript">
<!--
var slideShowSpeed = 2000

var crossFadeDuration = 3

var Pic = new Array()

Pic[0] = 'images/pano1.jpg'
Pic[1] = 'images/pano2.jpg'
Pic[2] = 'images/pano3.jpg'
Pic[3] = 'images/pano4.jpg'
Pic[4] = 'images/pano5.jpg'
Pic[5] = 'images/pano6.jpg'
Pic[6] = 'images/pano7.jpg'

var t
var j = 0
var p = Pic.length

var preLoad = new Array()
for (i = 0; i < p; i++){
   preLoad[i] = new Image()
   preLoad[i].src = Pic[i]
}

function runSlideShow(){
   if (document.all){
      document.images.SlideShow.style.filter="blendTrans(duration=2)"
      document.images.SlideShow.style.filter="blendTrans(duration=crossFadeDuration)"
      document.images.SlideShow.filters.blendTrans.Apply()      
   }
   document.images.SlideShow.src = preLoad[j].src
   if (document.all){
      document.images.SlideShow.filters.blendTrans.Play()
   }
   j = j + 1
   if (j > (p-1)) j=0
   t = setTimeout('runSlideShow()', slideShowSpeed)
}

//-->
</script> 

</head>

<body onload="runSlideShow()"   background="images/sfondo.gif">

<div align="center">
	<table border="0" width="985" cellspacing="0" cellpadding="0">
		<tr>
			<td align="center" width="977" colspan="3" height="150" style="padding: 5px">
			<p align="left">
			[img]images/geodatalogo.jpg[/img]
			[img]images/pano1.jpg[/img] <p align="left">
			</td>
		</tr>
		<tr>
			<td align="center" colspan="3">

</td>
		</tr>
		<tr>
			<td align="center" bgcolor="#FFFFFF" width="747" style="padding: 10px">
			<p class="MsoNormal" style="text-align: justify"></p>
			

</td>
			<td align="center" width="27"></td>
			<td align="center" bgcolor="#FFFFFF" width="197" style="padding: 10px">
			</td>
		</tr>
		<tr>
			<td align="center" colspan="3">
			<p align="right">
			[img]images/feed-icon32x32.png[/img]</td>
		</tr>
	</table>
</div>

<script type="text/javascript" language="javascript1.2" src="menu.js"></script> </body>

</html>