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>