I don't think Windows services have a GUI. They run in the background, not in the windowing system. The debugger shows you the GUI you have created, but only because you are running it from within QtCreator as an application and not a service, which runs it as part of the windowing system. When you install and run it as an actual service, it has no access to the windowing system.

Are you sure you really want to create a service? It sounds like you might be confusing the idea of a server (as in client-server) with service.