Prova questo .....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function activate(aRadioButton) {
if (aRadioButton.value == "0" && aRadioButton.checked) {
document.getElementById('my_text').disabled = true;
document.getElementById('my_text').value = "";
}
if (aRadioButton.value == "1" && aRadioButton.checked) {
document.getElementById('my_text').disabled = false;
document.getElementById('my_text').focus();
}
}
//-->
</script>
</head>
<body>
<input type="radio" value="0" name="r1" checked onclick="activate(this)">blocca
<input type="radio" value="1" name="r1" onclick="activate(this)">sblocca
<input type="text" size="40" id="my_text" disabled="true">
</body>
</html>