C#: GUI to display realtime messages from Windows Service

What you can do is have the windows service have way of registering for an event (you can do this through using Windows Communication Foundation). When your error comes up, it fires that event, and your winforms app will be notified. It's called a duplex contract:

http://social.msdn.microsoft.com/Forums/en-US/wcf/thread/0eb69998-0388-4731-913e-fb205528d374/

http://msdn.microsoft.com/en-us/library/ms731184.aspx

Actually the really cool thing is that you can have multiple applications listening this way too. So you can display it on screen, and have another application log it etc. without the two external apps knowing anything about each other.

I know this has already been mentioned, but use Windows Communication Foundation (WCF). Specifically, use the Publish-Subscribe Framework developed by Juval Lowy, author of Programming WCF Services. The details are described in this excellent MSDN article, and the source code is available for free at Lowy's website.

The neat thing about this framework is that it decouples the publisher, e.g., your Windows service, from any subscribers, e.g., your GUI. The publisher "publishes" events that are of interest to the Pub/Sub Service, which is always available. From the publisher's point of view, it doesn't matter if there are any subscribers or not. The Pub/Sub Service takes care of routing events to any and all registered subscribers. In this way, your Windows service publishes events as they occur, your GUI will subscribe/unsubscribe to the Pub/Sub Service when it loads/exits, and the Pub/Sub Service will notify your GUI as events occur.

I have used this setup in my project, and it works extremely well.

I've actually used the BitFactory Logger that has a socket logger that you can use for this purpose.