When Windows media player (WMP) is open, any attempt to open it again simply reactivates the existing WMP window. WMP is running as a single instance. It uses a relatively well known methods for this, creating a named mutex on startup and seeing if it already exists (by calling GetLastError and comparing with ERROR_ALREADY_EXISTS ). WMP uses a mutex called "Microsoft_WMP_70_CheckForOtherInstanceMutex" and this name seems to be consistent between WMP versions (at least starting from Windows...