codice:
<script>
function x(obj) {
f = obj.form
v = obj.options[obj.selectedIndex].value
for (var i=0;i<f.elements.length;i++) {
  el = f.elements[i]
  if (el.type=="checkbox") el.checked=(el.value!=v)
}
}
</script>
<form>
<select onchange="x(this)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>

<input type=checkbox name=pippo value="1">1

<input type=checkbox name=pluto value="2" checked>2

<input type=checkbox name=minni value="3" checked>2

</form>
Metterli in un form semplifica lo script...

ciao