Cosi.
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function pippo(ogg) {
document.getElementById('altro').disabled =true;
if (ogg.value == 'c') {
document.getElementById('altro').disabled =false;
document.getElementById('altro').focus();
}
} // function pippo()
//-->
</script>
</head>
<body>
<select onchange="pippo(this)">
<option value="a">A</option>
<option value="b">B</option>
<option value="c">C</option>
</select>
<input type="text" id="altro" name="altro" disabled>
</body>
</html>