RegisterWindowMessage can return different values for identical strings

RegisterWindowMessage can return different values for the same string between regular processes and windows services on the same physical machine. Additionally, PostMessage(HWND_BROADCAST, ...) will not work across desktops. From microsoft.public.vc.language:

Services run in a separate window station, and have their own desktop. Window messages can only be sent between windows on the same desktop. You won't be able to use window messages for communication between a regular process and a service... More...