it is more complicated than this. It works with PDFCreator 2.4.1 because it uses a different method to lookup the default application. While this fixes the situation on your system, we had the exact problem reported by other users for PDFCreator 2.4.1, so we tried to fix them, but still didn't manage to find a method that works for all supported OS.
You are correct, it isn't a problem in Adobe Reader, but rather a problem with .NET and Windows (and since PDFCreator uses .NET, also PDFCreator). Hopefully we will be able to find a method which works for everybody again soon.