Install Ajax with Visual Studio

Download Ajax here.

Then extract the Zip file.
Open Visual Studio.
Create / Open project ( Web based one )
Go to Toolbox , Right click -> Add new tab and give a name to it as you want.
Go to that tab -> Right click - > choose items.
Under .Net gramework components -> Browse -> browse the AjaxControlToolkit.dll and click the OK button to close the Choose Toolbox Items dialog.

Some time you might will have to face a security errors as below

  "AjaxControlToolkit.dll and click the OK button to close the Choose Toolbox Items dialog."

If then you have do is go to "C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE" and open "devenv.exe.config" file and change the

                        <NetFx40_LegacySecurityPolicy  enabled="true"/> 
                                          as below.
                    <NetFx40_LegacySecurityPolicy  enabled="false"/>

Now find some online tutorials Ajax + .Net. Happy Coding.