How to open a winform using windows service

A service is a long running executable that does not require user intervention. See here for more information[^].
As such it is not possible to open a Form from a service. And even if it is possible it is not recommended. Perhaps you want to make an application that communicates with a service? If that is the case Windows Communication Foundation[^] may be what you need.
Or maybe you need to rethink your strategy and ask yourself if you REALLY need a service :)

Be logical: a Windows Service keeps running even if nobody logged on. Where do you want to show a form? You are probably confused about the very concept of the service. However, there is nothing wrong in having a separate Windowed process which can interact with the Service. See the solutions by Abhinav and Naerling.

Moreover, this is a very typical approach used, in particular, for Service control.

By the way, the most adequate form of such application is the System Tray application based on the class System.Windows.Forms.NotifyIcon. See http://msdn.microsoft.com/en-us/library/system.windows.forms.notifyicon.aspx[^].

—SA