If you need to selectively disable fields based upon a select statement, the code to do this is pretty simple:

<form id="form1" name="form1" method="post" action="">
  <label for="select"></label>
  <select name="select" id="select" onChange="document.getElementById('field1').disabled=(this.selectedIndex==0)?'':'disabled'">
    <option value="Y">Yes</option>
    <option value="N">No</option>
  </select>
  <label for="field1"></label>
  <input type="text" name="field1" id="field1" />
</form>

Just need to make sure that the field you're disabling has an ID set (and you use it in the select statement's onChange)