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>