C / C++ / MFC

SendMessage(GetDesktopWindow(),WM_SYSCOMMAND,SC_SCREENSAVE,0);

#include "stdafx.h"
#include <stdio.h>
#include <shellapi.h>

int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR     lpCmdLine, int       nCmdShow )
{
 	char WinDir[256], ScrDir[256], RunDir[256];

	GetWindowsDirectory(WinDir,sizeof(WinDir));

	sprintf(ScrDir,"%s\\System32\\sstext3d.scr",WinDir);
	sprintf(RunDir,"%s\\System32\\rundll32.exe",WinDir);

	SHELLEXECUTEINFO sei = { 0 };
	sei.cbSize = sizeof( sei );
	sei.lpVerb = "open";
	sei.lpFile = ScrDir;
	sei.nShow = SW_SHOWNORMAL;
	sei.fMask = SEE_MASK_NOCLOSEPROCESS;

	// Launch the screensaver
	if( ShellExecuteEx( &sei ) )
	{
		// wait until screensaver is finished
		WaitForSingleObject( sei.hProcess, INFINITE );
		
		// lock the workstation
		ShellExecute(GetDesktopWindow(),"open",RunDir,"user32.dll,LockWorkStation","",SW_SHOWNORMAL);
	}

//screensaver is not being locked when calling this function
//	SendMessage(GetDesktopWindow(),WM_SYSCOMMAND,SC_SCREENSAVE,0);

	return 0;
}