<script>
function select_box(no) {
  if(no == 1){
   document.Form.List2.disabled = true;
    document.Form.List1.disabled = false;
  }else if(no == 2){
   document.Form.List1.disabled = true;
    document.Form.List2.disabled = false;
  }
}
</script>
<form name="Form">
 <input type="radio" value="1" name="r1" onClick="select_box(1)" />aaaa<br />
 <input type="radio" value="2" name="r1" onClick="select_box(2)" />AAAA<br />
<br />
<select name="List1">
 <option value="1">aaaa</option>
 <option value="2">bbbb</option>
</select>
<select name="List2">
 <option value="1">AAAA</option>
 <option value="2">BBBB</option>
  <option value="3">CCCC</option>
</select>
</form>