ho risistemato il form utilizzando il dom, spero daver capito e che cosi vada bene.
codice:
<HTML>
<HEAD>

<script language="JavaScript">
function convalidaForm(){

if(!document.getElementById('lista').checked && !document.getElementById('forfait').checked){
alert("Devi selezionare una opzione");
document.getElementById('errore').innerHTML='*';
return false;
}
if(!document.getElementById('AttNC').disabled){
if(document.getElementById('AttNC').value == 'Nessuna' || document.getElementById('AttNC').value == ''){
alert("Devi devi inserire una.....");
document.getElementById('AttNC').focus();
return false;
}
}
}
</script>
</HEAD>

<body> 

<form name="MyForm" action="altrapag.asp" method="post" onsubmit="return convalidaForm();">
<span id="errore"></span>
Lista:   <input type="checkbox"  id="lista" name="lista" value="Si" onClick="if(this.checked){document.getElementById('forfait').disabled=true;document.getElementById('forfait').checked=false}else{document.getElementById('forfait').disabled=false}">
Forfait: <input type="checkbox" id="forfait" name="forfait" value="Si" onClick="if(this.checked){document.getElementById('lista').disabled=true;document.getElementById('lista').checked=false}else{document.getElementById('lista').disabled=false}">

<select size="1" id="AttC" name="AttC" onChange="if(this.options.selectedIndex != 0){document.getElementById('AttNC').disabled=true;document.getElementById('AttNC').value='';}else{document.getElementById('AttNC').disabled=false;document.getElementById('AttNC').value='Nessuna'}">
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
   ... 
</select>
<textarea name="AttNC" cols="32" rows="5" id="AttNC" onFocus="if(this.value=='Nessuna')this.value=''" onBlur="if(this.value=='')this.value=defaultValue">Nessuna</textarea>
<input type="submit" name="Submit" value="Submit">
</form>

</body>
</html>