The latest versions of Windows Vista SP2 and Windows 7 will block the files downloaded from Internet, including the files extracted from the Zip archives downloaded from Internet. This problem does not occur if you extracted the files from the Zip archive using another tool than the default one from Windows Explorer.

To check if an extracted file was blocked, right click on that file from Windows Explorer and select Properties. If the file was blocked then an "Unblock" button and the message "This file came from another computer and might be blocked to help protect this computer" are displayed on the Properties tab.

In IIS from Vista SP2 and Windows 7 the DefaultAppPool application pool will run under a new and more restrictive identity named ApplicationPoolIdentity. This identity does not allow the "Full Trust" execution of the assemblies that were marked as downloaded from Internet and this can make some of our development libraries to not work as expected when used in ASP.NET applications.

To eliminate this restriction you can either unblock the assembly by clicking the "Unblock" button or you can change your application pool to run under a more permissive identity like Network Service.