I have a WCF service hosted in a Windows service.
This service the WCF have one metohd and in this method I have one important line :
Process Browser =
Under what user is the service running? Try running it under the currently logged in user, with privileges to interact with the desktop and see if that helps.
In general, its not a great idea to have services launching GUI processes. For example, what if no one is logged on. What if many people are logged on? Should it open in all sessions... etc. Have you considered exposing a simple (e.g. Net.NamedPipes) endpoint on your service, and writing a small client UI to interact with it?