You cannot have the Windows service with UI. Here is the hint for you: if it was possible, how could this UI work without desktop? When you log off Windows, the service keeps working, but there is no a desktop.
If you need to work some UI on some schedule (also not a very good idea), you could have a service which starts your application at certain moments of times, and only when it is possible. But such service already exist, called Windows Task Scheduler:
http://en.wikipedia.org/wiki/Windows_Task_Scheduler.
Please see my past answer:
Problem a Window Services.
—SA