asp.net Framework 2.0 ajax

Hello.

I initially made an ASP page. net and then I added
AJAX code by entering the page is the initial declaration <% @ Register
Assembly = "System.Web.Extensions, Version = 1.0.61025.0, Culture = neutral,
PublicKeyToken = 31bf3856ad364e35 "
Namespace = "System.Web.UI" TagPrefix = "asp"%> is the ScriptManager is in
Declaration web.config <add assembly = "System.Web.Extensions,
Version = 1.0.61025.0, Culture = neutral,
PublicKeyToken = 31bf3856ad364e35 "/></ assemblies>.

At the time gives me a problem SYS.

Like never??
How can I solve this problem.

Thanks to everyone who furnish a solution.
Bye
--
Prog.