<html>
<head>
<script language="JavaScript">
function abilita(){
window.nomeForm.info.style.visibility = "visible";
}

function disabilita(){
window.nomeForm.info.value = "";
window.nomeForm.info.style.visibility = "hidden";
}
</script>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body>
<form action="action" method="post" name="nomeForm">
Opzione 1<input name="scelta" type="radio" value="1" onClick="disabilita()">

Opzione 2<input name="scelta" type="radio" value="2" onClick="abilita()">

Opzione 3<input name="scelta" type="radio" value="3" onClick="disabilita()">

<input name="info" type="text" size="10" maxlength="10" style="visibility:hidden">

<input name="submit" type="submit" value="Invia">
</form>
</body>
</html>