It is possible, in principle, to set the property "enable the service to
interact with the desktop", which would allow it to show a user interface.

   However, this is not recommended. The "normal" way of providing a UI for
a Service is to write an ordinary Windows application to show the interface,
and then have this application interact with the service by means of some
method of interprocess comunication, such as .Net Remoting, for instance.

Diego,

   You don't.  You should not have a service try and use user interface
elements.  Rather, you should have the service expose an endpoint (through
remoting or WCF) and have a separate program access that endpoint in order
to display things to the user about the service.

   Hope this helps.