Error on printing documents from Interop/c#

I have made a library in c# for printing word documents with
Microsoft.Office.interop.Word.
When this library is used from a Windows Forms application the
printing works wel. But when I try to use this library within a
windows service there is a exception on the methos
Microsoft.Office.interop.Word._Document.PrintOut, says 'No printer
installed'.
Before print I set the ActivePrinter property to the name of the
printer.
I have tried to start the service as a windows user and not a
localsystem but the results is the same.
Someone can help?
Many thanks

Edika