System.Windows.Interactivity: Could not load file or assembly 'System.Windows, Version=2.0.5.0

If you have a WPF project you need to use the WPF specific version of that assembly, maybe you took the wrong .dll?

Here are the two respective file paths on my system:

C:\Program Files (x86)\Microsoft SDKs\Expression\Blend\.NETFramework\v4.0\Libraries\System.Windows.Interactivity.dll
C:\Program Files (x86)\Microsoft SDKs\Expression\Blend\Silverlight\v4.0\Libraries\System.Windows.Interactivity.dll

You need to reference the dll's in the main project (not just the control libraries). It was the problem in my case. See more info here: Could not load file or assembly 'System.Windows.Interactivity'