For one reason or another, several of our Revit 2011 installs cause Windows not to display the thumbnail preview of Revit project (.rvt) and Families (.rfa). After speaking with Adsk support, they determined the problem to be with Windows and not Revit. The fix is pretty easy.
You first need to open a DOS Window in Windows. From the Windows Start menu, click Run and then type "cmd" in the Run dialog. This will open a DOS Window. In the command line then type:
regsvr32 "C:\Program Files\Autodesk\ Revit Architecture 2011\Program\Revit.FilePreview.dll"
The Revit.FilePreview.dll will register the DLL library for previews. If everything registered successfully, a successful registration of the DLL module should appear and the thumbnail previews will appear.