You need to develop a different application, Windows Service. It cannot carry any functionality of Windowed application. In general case, the service works when there is no any desktop, so no UI is possible. If you want some UI supporting your service, it should be yet another application which communicates with your Service using some IPC, for example, named pipes, which can also be done using "classical" remoting or WCF (self-hosted in your service), both can use, for example, the same named pipes mechanism via so-called "IPC" channel.
Please see:
http://en.wikipedia.org/wiki/Windows_Service[
^],
http://technet.microsoft.com/en-us/library/cc783643%28v=ws.10%29.aspx[
^],
http://msdn2.microsoft.com/en-us/library/ms685141.aspx[
^].
If your existing application has thoroughly isolated UI, you can possible refactor it the way to extract some functionality into separate library assemblies, to be used in both service and the Windows application.
—SA
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