Could not load file or assembly ‘AjaxControlToolkit’ or one of its dependencies. Access is denied.

I have solved my problem deleting my temporary files in both .net frameworks folders

C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files

For 64bit systems, also check the same under the Framework*64* folders i.e.

C:\Windows\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files