codice:
<html>
<head>

<script type="text/javascript"> 
function no_sponsor(){ 
setTimeout('document.getElementById("sponsor").style.display = "none"',5000);}

function no_sponsor_subito(){ 
document.getElementById("sponsor").style.display = "none";}  

</script> 

</head>

<body onload="no_sponsor()">


<div id="sponsor" style="text-align: Center; position: Absolute; top: 50%; left: 50%; width: 500px; height: 200px; margin: -100px, -190px; visibility: visible;">
<center>
<table border="1" width="100%" >
  <tr>
<td width="100%" height="20">
<p align="right">Chiudi [x]</p></td>
  </tr>
  <tr>
    <td width="100%" height="190"><center>[img]spot.gif[/img]</td>
  </tr>
</table>      
</center>        
</div>


</body>

5000 naturalmente è il numero di millisecondi.
Ciauuuu!

</html>