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>