Cosi:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
</head>
<body>
<form name="myform">
  


    <input name="NomeRadio" type="radio" id="No" value="NO" checked="checked" onclick="document.getElementById('Nometextarea').style.display='none';">
  NO
  <input type="radio" name="NomeRadio" id="Si" value="SI" onclick="document.getElementById('Nometextarea').style.display='block';">
  SI 
  <textarea name="Nometextarea" cols="45" rows="5" id="Nometextarea" style="display:none"></textarea>
  

  </p>
</form>
</body>
</html>