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Don't mess with the screensaver settings, use SetThreadExecutionState. This is the API for informing windows on the fact that your application is active:
Enables an application to inform the system that it is in use, thereby preventing the system from entering sleep or turning off the display while the application is running.
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Multimedia applications, such as video players and presentation applications, must use ES_DISPLAY_REQUIRED when they display video for long periods of time without user input