Fix "Source Files could not be Found or Downloaded" in Windows 8 and Later

Windows 8, 8.1, Server 2012 and 2012 R2 come with only .NET Framework 4.5 fully installed. However, some apps such as CTI Navigator Desktop require that some components of an earlier version of .NET Framework (such as 3.5, 3.0 or 2.0) also be installed in order to run properly.  Normally, if an app needs a .NET Framework feature that is not already installed, Windows will automatically prompt you to install the missing Windows feature over the Internet. If so prompted, select "Install this feature."

Otherwise, you can manually enable .NET Framework 3.5 (which includes code from earlier versions) as follows.

To enable the .NET Framework 3.5 in Control Panel

  1. In Windows 8.1:  Right-click on the Start icon and select Control Panel.
    In Windows 8: On the Start screen, type and then choose Control Panel. (To access the Start screen, press the Windows logo key on your keyboard.)
  2. Choose Programs (or Programs and Features), and then choose Turn Windows features on or off.
  3. Select the .NET Framework 3.5 (includes .NET 2.0 and 3.0) check box so the box displays a black square (indicating that only part of the feature is enabled).  
    Note: The two “Windows Communication Foundation sub-features in .NET Foundation 3.5 do not need to be activated. 
  4. Click OK.

Error Fixes

Some Windows security updates (such as KB2966826, KB2966827, and KB2966828) can interfere with installing earlier versions of .NET Framework. This can result in error messages such as

  • 0x800f0906:   "The source files could not be downloaded...";  or
  • 0x800F081F: "The source files could not be found...).
A "Fix it" file can be downloaded from the Microsoft Download Center to resolve such errors. Select the package appropriate for your operating system type: 32-bit or 64-bit. (System Type is indicated  under System in the Windows Control Panel.)  

See Microsoft update 30005628 for details regarding this problem.