AAAARGGGGGGGGHHHHHHHH.....
Ok, the error has been seen quite often...I've checked a lot of forums and all they seem to provide as a solution is to uninstall everything and make sure the latest dll's are in the bin directory of my website.
Here are the files in my bin directory:
AjaxControlToolkit.dll version: 1.0.61214.0
AjaxControlToolkit.pdb version:
AJAXExtensionsToolbox.dll version: 1.0.61025.0
System.Web.Extensions.Design.dll version: 1.0.61025.61025
System.Web.Extensions.dll version: 1.0.61025.61025

Now, I've done exactly that! and checked all the versions but still the error remains....
Below you see my web.config file

Im not sure about this part in the web.config, but if I remove it I receive a bunch of other errors about the <microsoft.web> section:
    <sectionGroup name="microsoft.web" type="Microsoft.Web.Configuration.MicrosoftWebSectionGroup, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
      <section name="converters" type="Microsoft.Web.Configuration.ConvertersSection"/>
    </sectionGroup>


WEB.CONFIG


<?xml version="1.0"?>

<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
 <configSections>
    <sectionGroup name="microsoft.web" type="Microsoft.Web.Configuration.MicrosoftWebSectionGroup, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
      <section name="converters" type="Microsoft.Web.Configuration.ConvertersSection"/>
    </sectionGroup>
    <sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">

   <sectionGroup name="scripting" type="Microsoft.Web.Configuration.ScriptingSectionGroup, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
    <sectionGroup name="webServices" type="Microsoft.Web.Configuration.ScriptingWebServicesSectionGroup, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
     <section name="jsonSerialization" type="Microsoft.Web.Configuration.ScriptingJsonSerializationSection, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>
     <section name="profileService" type="Microsoft.Web.Configuration.ScriptingProfileServiceSection, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>
     <section name="authenticationService" type="Microsoft.Web.Configuration.ScriptingAuthenticationServiceSection, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>
    </sectionGroup>
   </sectionGroup>
  </sectionGroup>
 </configSections>
 <system.webServer>
  <validation validateIntegratedModeConfiguration="false"/>
  <modules>
   <add name="ScriptModule" preCondition="integratedMode" type="Microsoft.Web.UI.ScriptModule, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
  </modules>
  <handlers>
   <remove name="WebServiceHandlerFactory-ISAPI-2.0"/>
   <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="Microsoft.Web.Script.Services.ScriptHandlerFactory, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
   <add name="ScriptResource" verb="GET" path="ScriptResource.axd" type="Microsoft.Web.Handlers.ScriptResourceHandler"/>
  </handlers>
 </system.webServer>
 <microsoft.web>
  <converters>
   <add type="Microsoft.Web.Script.Serialization.Converters.DataSetConverter"/>
   <add type="Microsoft.Web.Script.Serialization.Converters.DataRowConverter"/>
   <add type="Microsoft.Web.Script.Serialization.Converters.DataTableConverter"/>
  </converters>
 </microsoft.web>

 <connectionStrings>
  <add name="SQLAuth" connectionString="data source=LT1600\SQLEXPRESS;Initial Catalog=Data;User Id=sa;Password=hallow;" providerName="System.Data.SqlClient"/>
  <add name="AdminLink" connectionString="data source=LT1600\SQLEXPRESS;Initial Catalog=Data;User Id=sa;Password=hallow;" providerName="System.Data.SqlClient"/>
  <add name="WebUser" connectionString="data source=LT1600\SQLEXPRESS;Initial Catalog=Data;User Id=webuser;Password=hallow;" providerName="System.Data.SqlClient"/>
 </connectionStrings>
 <system.web>
  <trust level="Medium"/>

  <compilation debug="true" strict="false" explicit="true">
   <assemblies>
    <add assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/></assemblies>
  </compilation>
  <pages maintainScrollPositionOnPostBack="true">
   <controls>
    <add tagPrefix="asp" namespace="Microsoft.Web.UI" assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add tagPrefix="asp" namespace="Microsoft.Web.UI.Controls" assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" tagPrefix="ajaxToolkit"/>
   </controls>
   <tagMapping>
    <add tagType="System.Web.UI.WebControls.CompareValidator" mappedTagType="Microsoft.Web.UI.Compatibility.CompareValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add tagType="System.Web.UI.WebControls.CustomValidator" mappedTagType="Microsoft.Web.UI.Compatibility.CustomValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add tagType="System.Web.UI.WebControls.RangeValidator" mappedTagType="Microsoft.Web.UI.Compatibility.RangeValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add tagType="System.Web.UI.WebControls.RegularExpressionValidator" mappedTagType="Microsoft.Web.UI.Compatibility.RegularExpressionValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add tagType="System.Web.UI.WebControls.RequiredFieldValidator" mappedTagType="Microsoft.Web.UI.Compatibility.RequiredFieldValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    <add tagType="System.Web.UI.WebControls.ValidationSummary" mappedTagType="Microsoft.Web.UI.Compatibility.ValidationSummary, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
   </tagMapping>
   <namespaces>
    <clear/>
    <add namespace="System"/>
    <add namespace="System.Collections"/>
    <add namespace="System.Collections.Specialized"/>
    <add namespace="System.Configuration"/>
    <add namespace="System.Text"/>
    <add namespace="System.Text.RegularExpressions"/>
    <add namespace="System.Web"/>
    <add namespace="System.Web.Caching"/>
    <add namespace="System.Web.SessionState"/>
    <add namespace="System.Web.Security"/>
    <add namespace="System.Web.Profile"/>
    <add namespace="System.Web.UI"/>
    <add namespace="System.Web.UI.WebControls"/>
    <add namespace="System.Web.UI.WebControls.WebParts"/>
    <add namespace="System.Web.UI.HtmlControls"/>
    <add namespace="Samples.AccessProviders"/>
   </namespaces>
  </pages>
  <httpHandlers>
   <remove verb="*" path="*.asmx"/>
   <add verb="*" path="*.asmx" validate="false" type="Microsoft.Web.Script.Services.ScriptHandlerFactory, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
   <add verb="GET" path="ScriptResource.axd" type="Microsoft.Web.Handlers.ScriptResourceHandler" validate="false"/>
  </httpHandlers>
  <httpModules>
   <add name="WebResourceCompression" type="Microsoft.Web.Handlers.WebResourceCompressionModule, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
   <add name="ScriptModule" type="Microsoft.Web.UI.ScriptModule, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
  </httpModules>

 </system.web>

</configuration>

Hi Peter,

Are you using the Release Candidate of Ajax? If so, then all of the namespaces have changed from Microsoft.* to System.*. You just need to search & replace within your web.config, or, cut n paste from the template one in the Microsoft ASP.NET folder. 

Regards,
Chris

Please Post Detail About this error. Because meet error

 Thank .............
 

Ok I Don't know why it's working now for me but here what i did....

1. Install the latest Asp.NET Ajax

2.Open my web.config in my production server and replace this line

by

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

It's the same thing but the PublicKeyToken is in capitals....

Save it and browse to my site and bingo!

Hope this help...

Did this fix actually work for anyone???

 AJAX 1.0 is working beautifully on my developement machine but when I publish a website to my production server I get:

Configuration Error

Line 36: 		<compilation debug="true">
Line 37: 			<assemblies>
Line 38: 				<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
Line 39: 				<add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
Line 40: 				<add assembly="System.Web.Extensions.Design, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/></assemblies>

 I have been on Microsoft's "off-shore" support line for two hours.  They walked me through installing "hotfix for issue 915782. ".  After two hours the support technician told me that this hotfix doesn't really fix this problem and if I would like to open another paid support case they would be happy to send this problem to the engineers and research it.  The support tech told me that currently do not have a fix for this problem.

 
I really like the functionallity of AJAX and it would be a great addition to our corporate public website BUT like all Microsoft products I fear I am going to have to wait for AJAX SP1 or AJAX SP2 before it actually works.  With as long as I have been using Microsoft products you think I would know better by now.  Maybe I will try Oracles "Comet" or Suns "Soft Scrub" technology!
 

I meant "did" you run the .msi installer on the server. ;-)

If you didn't, you need to.  (Barring that, you can copy the DLLs into the /bin folder as long as you can configure your app for full trust.)

I am having the same problem.

I am trying to run the AJAX Documentation website. I am trying to follow the readme.htm file. I have followed all of it's instructions.

I am getting the same error,

Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

Sorry for my ignorance, but which dll's and into which bin folder?

Thanks, Dan R.

Where is this file located and what if I don't have it, where would I get, would I get it by the process of installing something or do I need to pull if from somewhere?

I have installed:

VB 2005 EE

Visual Web Developer EE

SQL Server Express 2005 EE

Management Studio Express

 Oh, duh, do I need to install the AJAX control toolkit at this point?

Is that why I don't have a 'Microsoft ASP.NET directory under 'C:/Program Files'?

You need to install ASP.NET 2.0 AJAX Extensions 1.0

http://www.microsoft.com/downloads/details.aspx?FamilyID=ca9d90fa-e8c9-42e3-aa19-08e2c027f5d6&displaylang=en

Never mind I just answered my own question, I hadn't installed the extensions yet, that's why 'microsoft asp.net' directory didn't exist or the system.web.extensions.dll file, this is all so complicated, thanks,

Dan Ros.

Hey there,

One quick thing worth noting is that even AFTER I installed the AJAX extensions, I still got this error. I ended up having to restart the IIS service on the local machine...