You need to call the underlying WinAPI. If you need special things of an operating system you have to use the underlying OS API (Windows => WinAPI, Linux ..., Mac OS ...)

As fatjuicymole has already been written, use:
Qt Code: 
  1. #if defined(Q_WS_WIN)
  2.         SystemParametersInfo(SPI_SETSCREENSAVERRUNNING, TRUE, &bOldState, 0);
  3. #endif
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Best Regards
NoRulez