I am using the  ASP.NET Ajax Extensions 1.0
 Below is my aspx and codebehind. There is literally nothing on my page.
When I run my page I get a Javascript error 'Sys' is undefined.
 Anyone have any idea why?
 
<%@ Page Language="C#" MasterPageFile="~/MasterPages/Main.Master" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="_Default" %>
<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
Namespace="System.Web.UI" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<asp:ScriptManager id="ScriptManager1" runat="server">
</asp:ScriptManager> 
 
<asp:UpdatePanel id="UpdatePanel1" runat="server"> 
<ContentTemplate>
 
</ContentTemplate>
</asp:UpdatePanel>
 
</asp:Content>
 
Below is my codebehind
 
 
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
 
 
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
}
 

Do you have the proper items in your webconfig file?
You need
<add verb="GET" path="ScriptResource.axd" type="Microsoft.Web.Handlers.ScriptResourceHandler" validate="false"/>

in 
<system.web>
   <httpHandlers>

Here is my Web.config I still get an the same Javascript error
 

<?xml version="1.0"?><configuration>


<appSettings/>

<connectionStrings/>


<system.web>

<!-- 

Set compilation debug="true" to insert debugging 
symbols into the compiled page. Because this 
affects performance, set this value to true only 
during development.
-->

 
<httpHandlers>

<add verb="GET" path="ScriptResource.axd" type="Microsoft.Web.Handlers.ScriptResourceHandler" validate="false"/>

</httpHandlers>

<compilation debug="true">

<assemblies>

<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

<add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>

<add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/></assemblies></compilation>

<!--
The <authentication> section enables configuration 
of the security authentication mode used by 
ASP.NET to identify an incoming user. 
-->

<authentication mode="Windows"/>

<!--
The <customErrors> section enables configuration 
of what to do if/when an unhandled error occurs 
during the execution of a request. Specifically, 
it enables developers to configure html error pages 
to be displayed in place of a error stack trace.
<customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
<error statusCode="403" redirect="NoAccess.htm" />
<error statusCode="404" redirect="FileNotFound.htm" />
</customErrors>
-->

</system.web>

</configuration>

remove 
<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
Namespace="System.Web.UI" TagPrefix="asp" %>

Add below code  between <system.web> element in web.config

<pages>

<controls>

<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

</controls>


</pages>


<compilation debug="false">

<assemblies>

<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

</assemblies>

</compilation>

<httpHandlers>

<remove verb="*" path="*.asmx"/>

<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

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

</httpHandlers>

<httpModules>

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

</httpModules>

</system.web>

Good luck

Try the fix mentioned in this blog posting if you are using Windows 2003 as your web server -
http://aspadvice.com/blogs/sswafford/archive/2007/01/25/ASP.NET-AJAX-v1.0-Sys-is-undefined-error.aspx