Prova cosi....

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

<html>
<head>
<title>Untitled</title>
</head>
<body>

<script language="JavaScript" type="text/javascript">
<!--
function protect(anObject, protection) {
 if (anObject == null) {
  return true;
 } // if (anObject == null)
 var members = anObject.children.length;
 var i = 0;
 for (i=0;i<members;i++) {
  var curObject = anObject.children.item(i);
  if (curObject != null) {
   protect(curObject,protection);
	} // if (curObject != null)
 } // for (i=0;i<=members;i++)
 anObject.disabled = protection;
 return true;
} // function protect(anObject, protection)
//-->
</script>
<div id="mydiv">
                    <input type="radio" name="anticipo" value="0" checked disabled >
                    0%     
                    <input type="radio" name="anticipo" value="10" disabled >
                    10%     
                    <input type="radio" name="anticipo" value="20" disabled >
                    20%     
                    <input type="radio" name="anticipo" value="30" disabled >
                    30%     
                    <input type="radio" name="anticipo" value="40" disabled >
                    40%     
                    <input type="radio" name="anticipo" value="50" disabled >
                    50%
</div>										
                    <input type="checkbox" name="anticipo_abi" value="quota" onclick="protect(document.getElementById('mydiv'), ! this.checked)">
                    ABILITA QUOTA: 
                    <input name="anticipo" type="text" id="anticipo_q" disabled>

</body>
</html>