Powerpoint Interop在Windows服务中失败，但在Windows窗体应用程序中工作正常

I've written an application to watch a folder, and whenever a PowerPoint document is dropped into it, it goes through the slides within it and generates png images of each slide. This works fine in a windows forms application but I've put exactly the same code in a windows service, used the same test file and it fails to open it with the following error:I've written an application to watch a folder,