Best strategy to separate/connect Business Logic and GUI?

I am designing a desktop application that is basically a control system. As such, the Business Logic is the most important. I would like this to load and run as a system process. I would also like to have a GUI, which can be loaded and closed at will.
The question is, what is the best choice of technology platfor for both, and what IPC/API is best for having the BL communicate with the GUI?
(by the way, the BL must be able to comfortably use ActiveX).
Thanks