Puoi fare cosi

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

<html>
<head>
<title>Untitled</title>
</head>
<body>
<input type="radio" name="r" checked onclick="document.getElementById('t').disabled=true"/>Off

<input type="radio" name="r" onclick="document.getElementById('t').disabled=false;document.getElementById('t').focus();"/>On

<textarea id="t" rows="10" cols="50" disabled></textarea>
</body>
</html>