thumbnail generation in windows service

I try to create thumbnails for pdf files using the microsoft windowns 7 api codepack.

If I use this code from a normal desktop application everything works fine.

But if I use it from a windows service, to create thumbnails for our document manager, I get the folowing Exception:

System.NotSupportedException: The current ShellObject does not have a valid thumbnail handler or there was a problem in extracting the thumbnail for this specific shell object. ---> System.Runtime.InteropServices.COMException: Klasse nicht registriert (Ausnahme von HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG))

With Word and Excel files it also works inside the windows service, so I think its a problem with Acrobat Reader.

What can I do to fix the problem?