Hi,

Based on my understanding, the System.Windows.dll is related to Silverlight and not WPF. Therefore, referencing a Silverlight project in your WPF application could be the cause of the issue. Remember that you can't reference Silverlight projects in WPF. If this is not the case, you could try one of the followings too:

• Clear the reference to the System.Windows assembly and reference it again.
• Check the GAC to see if System.Windows is registered. If not, you will have to register it manually to the GAC.
  

Hope this helps,

Federico Martinez
http://blogs.southworks.net/fmartinez