I am sure I asked this question a week or so ago, but I can’t find it so asking again.
there is no function which permanently sets PDFCreator as default printer.
A possible cause for this is a user having set the PDFCreator to be default printer at a earlier point and not selected a different default printer since then. (Even if PDFCreator was removed for some time, windows might still recognize it as default printer in a later setup)
This happens, when you are using the context menu in the explorer to convert files. As this does print the file behind the scenes, it requires that PDFCreator is the default printer or the result will be printed to a different printer.
Somehow, I think this is associated with my problem. However, the users do not use PDFCreator for anything but output from our software. And the problem of it becoming the default printer is happening on workstations that do not even invoke the pdfcreator driver.