sys Undefine Error in java script

<script src="jquery-1.8.3.js" type="text/javascript"></script>

   <script src="jquery-1.8.3.min.js" type="text/javascript"></script>

   <script type="text/javascript" language="javascript">
           Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
           Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
           function BeginRequestHandler(sender, args) {
               var elem = args.get_postBackElement();
               ActivateAlertDiv('visible', 'AlertDiv');
               document.body.style.cursor = 'wait';
               //Disable control during postabck
               pbControl = args.get_postBackElement();  //the control causing the postback
               pbControl.disabled = true;
           }
           function EndRequestHandler(sender, args) {
               ActivateAlertDiv('hidden', 'AlertDiv');
               document.body.style.cursor = 'auto';
               //Enable control during postabck
               pbControl.disabled = false;
               pbControl = null;
           }
           function ActivateAlertDiv(visstring, elem) {
               var adiv = $get(elem);
               adiv.style.visibility = visstring;
           }


   </script>