codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<SCRIPT>
<!-- Validate entries

function valid() {
 var control=document.theform.control;
 if (control.checked==false){
  alert("Attenzione!! Verificati i dati prima di andare AVANTI...")
  document.theform.control.focus()
  return false
 }
 return true;	
}

// -->
</SCRIPT>


</head>
<body>

<FORM METHOD="POST" name="theform" action="verifica_form.asp" onsubmit="return valid(this)">
........
<input type="checkbox" name="control" value="SI">
........
<input type="submit" value="   Avanti   " name="B1" >
</FORM>
</body>
</html>