Try this:
LaunchExecutable ("C:\\WINDOWS\\system32\\yourScreenSaver.scr //s");
If you want to activate the defined screen saver in the system instead of a fixed one, you need to retrieve first the path to the screen saver executable:
RegReadString (REGKEY_HKCU, "Control Panel\\Desktop", "SCRNSAVE.EXE", file, MAX_PATHNAME_LEN, &stringSize);