You can't show a form from a Windows Service, and you can't run a GUI application from a Windows Service. The reason is that a service can be running while no user is logged in. What you *should* do is create a system tray application (configure it to run when a user logs in), and then you can have the app notify the service when it's available, allowing the service to push notifications to it.
I wrote an article that has a windows service communicating with a system tray app. It may or may not help you:
Synchronicity - A Folder Synchronizing Application[
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