En Checkbox skal enables...

<input type="checkbox" id="leader"><br><br>
<input type="checkbox" id="o0"><br>
<input type="checkbox" id="o1"><br>
<input type="checkbox" id="02">

<script language="JScript">

function l_down(){
  if (document.getElementById("leader").checked == true) {
      document.getElementById("o2").disabled = true;
  }
  else {
      document.getElementById("o2").disabled = false;
  }
}

document.getElementById("leader").onclick = l_down;
</script>

tak