always keep the window in the center when move the scroll bar
<style>
  #Window {
 display: none;
 text-align: center;
 border: 1px solid #333;
  position:absolute;
  width: 200px;
  height: 80px;
  z-index:9999;
  background:#fff;
  border-radius: 5px;
  padding: 10px;
     }
  </style>

  <script>
      $("#window").css('top', 200);
     $("#window").css('left', winW/2- $("#window").width());
   $(window).scroll(function () {

         var winH = $(window).height();
          setTimeout( function(){
   $('#alertWindow').animate({'top':winH/2-$("#window").height()/2},200);
 }, 1000);
      });
  </script>

 <div id="window">this is scroll window</div>

$( ".myDialogWindow" ).dialog( "option", "position", 'center' );