I'm planning to do this by dividing the core part of the application into a Windows service, and then having the front end communicate with it using .NET Remoting over named pipes. This means it will be possible for the front end to connect to to the service from any session on the same machine, and at this stage I'm not planning to add any authorisation controls because I think that's good enough.

Not everybody wants to run as a service, so I'll also need to make sure the core part can run in the front end process as it does now.