This is only true if you manually stop the playback or do some other user action. If the movie playback terminates because it reaches the movie's end, this is no activity for Windows.
Michael
OK, but from what you said "PS++ only handles standby, i.e prevents the PC from going to S3 or S4 while it is busy. This is done by calling SetThreadExecutionState (ES_SYSTEM_REQUIRED|ES_CONTINUOS) e.g. as long as a movie is beeing played" it would appear that PS++ calls something whilst a movie is being played that tells Windows it's busy, which should mean that Windows will only go into standby xx minutes (where xx is set in Windows Power Options) after it last received such a "busy" message, so whether the movie playback terminating automatically because it reached the end counts as activity should be irrelevant.