In visual studio 2008 WPF projects sometimes you will get this error message when you are trying to view the xaml designer.
After little bit of search I found the solution. First update the visual studio 2008 to service pack 1 then go to the Visual Studio 2008 Command Prompt (Start->All Programs ->Microsoft Visual Studio 2008->Visual Studio Tools)
Then type devenv /ResetSkipPkgs the it will load the packages. Hope this will help….