'Sys' is undefined error in visual studio 2005

 <system.web>
 <httpHandlers>
<add  verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />


add this in web config.