Updated solution for previews not working in Revit:
The following solution was found in a discussion thread on augi.com
Click Vista's Start button
In the Search box, type Cmd
Right-click on Command Prompt in the search result listing
Click “Run as Administrator"
Type regsvr32 "c:\program files\revit architecture2009\program\revitpreview.dll"
The above "correctly" registers revitpreview dll.
Don't forget the ...... " "