<script>
if (document.layers)
  document.captureEvents(Event.KEYDOWN);
document.onkeydown =
  function (evt) {
    var keyCode = evt ? (evt.which ? evt.which : evt.keyCode) : 
event.keyCode;
    if (keyCode == 13) 
    {
     
    MM_validateForm('username','','R','password','','R');
    return document.MM_returnValue
      return false;
    }
    else
      return true;
  };

</script>