I need to run a service interactively on my computer.
(I am already aware of the implications.)
How do I enable a service (running in session 0) to run interact with the current desktop in Windows 7?
As of Windows Vista, there are no directly interactive services. "Allow service to interact with desktop" is basically just a legacy option, although if you use it UI0Detect should pop up and allow you to switch to a special desktop to interact with the service.
This article describes some options. If you have further questions, I suggest you ask at Stack Overflow as this is a programming problem.
I realize the original question was about a year ago, however it would be useful if more people are aware of how and why the question itself is incorrect.
You're confusing a service with an application; they are different beasts. What you actually want to do is create a separate application which you interact with and that exchanges information with your service.
Please read this article: http://asprosys.blogspot.com/2009/03/allow-service-to-interact-with-desktop.html