var ButtonID;
   Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(DisableButton);
   Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EnableButton);
       function EnableButton(sender, args){
           var oControl = document.getElementById(ButtonID); 
           if (oControl!=null) {
               if (oControl.type=="submit") oControl.disabled = false;
           }
       }
       function DisableButton(sender, args){
           var oControl = args.get_postBackElement(); 
           if (oControl.type=="submit") {
               ButtonID=oControl.name;
               oControl.disabled = true; 
           }
       }