I have not seen this error before, but after some investigation I managed to find some info. Most probably this is caused because you are missing Microsoft.Windows.Design.Extensibility.dll, it should be situated in
C:\Program Files (x86)\Microsoft Visual Studio 9.0\Common7\IDE\PublicAssemblies, if you don't have it there maybe you have something corrupted with .NET Framework installation. Another thing I can advise you is to install Blend Expression. As far as i know Blend installation should install Microsoft.Windows.Design.Extensibility.dll . If installing Blend itself doesn't help try to copy this dll to the path mentioned above.
Hope this helps.
All the best,
the Telerik team
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