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

Today i have got this nasty error, i fixed it and then decided to share it with you people. I was working on a website developed in version 2.0 but when i tried to run it through my friend's system (i had given a copy of website to him) then face this error.
To fix this error i simply installed ajax in that system and website come to life.

To solve issue without installing AJAX.NET:

Copy AjaxControlToolkit.dll and AjaxControlToolkit.pdb version 1.0.61025.0 from my system (in which ajax is installed) to ASP.NET App bin folder of my friend's system.

(i install my AjaxControlToolkit in C:\Program Files\Microsoft ASP.NET\ASP.NET 2.0 AJAX Extensions\AjaxControlToolkit)

Copy System.Web.Extensions.dll (C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions) and System.Web.Extensions.Design.dll (C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions.Design) from my system to ASP.NET App bin folder of my friend's system.

Run the website and it's working fine.