codice:
function CheckAll()
   {
      for (var i=0;i<document.pippo.elements.length;i++)
      {
         var e = document.pippo.elements[i];
         if (e.name != "allbox")
            e.checked = document.pippo.allbox.checked;
      }
   }




<form name="pippo" id="pippo" action="pippo.asp">
<input name="allbox" type="checkbox" id="allbox" value="" onClick="CheckAll();">
<input name="uno" type="checkbox" id="uno" value="ciao">
</form>