Assembly 'AjaxControlToolkit, Version=3.0.30512.20315, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' uses 'System.Web.Extensions, Version=3.5.0.0,

Error Assembly 'AjaxControlToolkit, Version=3.0.30512.20315, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' uses 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' which has a higher version than referenced assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

The solution worked for me is...
1) I changed target framework to 3.5 in all the projects in my solution
2) Clean all the projects in the solution
3) Rebuild the full solution
4) Open the web.config file and remove the content
<runtime >
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35">
<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0">
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35">
<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0">
</dependentAssembly>
</assemblyBinding>
</runtime>
and save.
5) Now close the Visual Studio 2008 and open again, clean the full solution and rebuild.
oooorey It worked :)
Hope this may help someone.

Visual Studio 2008 - The requested page cannot be accessed because the related configuration data for the page is invalid

Error

---------------------------
Microsoft Visual Studio
---------------------------
Unable to start debugging on the web server.

Server Error

Internet Information Services 7.0

Error Summary

HTTP Error 500.19 - Internal Server Error

The requested page cannot be accessed because the related configuration data for the page is invalid.

Detailed Error Information

Error Summary
HTTP Error 500.19 - Internal Server Error
The requested page cannot be accessed because the related configuration data for the page is invalid.

Config Source

   -1:
    0: 

Make sure the server is operating correctly. Verify there are no syntax errors in web.config by doing a Debug.Start Without Debugging. You may also want to refer to the ASP.NET and ATL Server debugging topic in the online documentation.
---------------------------
OK Help
---------------------------

Solution
You are using IIS 7.0 on a computer that is running Windows Vista. Additionally, you configure the Web site to use UNC Passthrough authentication to access a remote Universal Naming Convention (UNC) share.The IIS_IUSRS group does not have the appropriate permissions for the ApplicationHost.config file or for the Web.config file.

I have given rights to folder which contains the web.config file. Now it works for me...

Types and there definitions in C#.NET

Type

Base class that represents any type.

Value Type

Base class that represents any value type.

Reference Types

Any data types that are accessed through a reference and stored on the heap.

Built-in Value Types

Includes most of the standard primitive types, which represent numbers, Boolean values, or characters.

Enumerations

Sets of enumerated values.

User-defined Value Types

Types that have been defined in source code and are stored as value types. In C# terms, this means any struct.

Interface Types

Interfaces.

Pointer Types

Pointers.

Self-describing Types

Data types that provide information about themselves for the benefit of the garbage collector .

Arrays

Any type that contains an array of objects.

Class Types

Types that are self-describing but are not arrays.

Delegates

Types that are designed to hold references to methods.

User-defined Reference Types

Types that have been defined in source code and are stored as reference types. In C# terms, this means any class.

Boxed Value Types

A value type that is temporarily wrapped in a reference so that it can be stored on the heap.