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procedure DrawTextOnDesktop(TextToDraw: string) ;
 var
    Handle: HWND;
    Dc: HDC;
    ACanvas: TCanvas;
 begin
    Handle := GetDesktopWindow;
    Dc := GetWindowDC(Handle) ;
    ACanvas := TCanvas.Create;
    try
      ACanvas.Handle := DC;
      BeginPath(ACanvas.Handle) ;
      ACanvas.Font.Color := clRed;
      ACanvas.Font.Name := 'Tahoma';
      ACanvas.Font.Size := 30;
      SetBkMode(ACanvas.Handle, TRANSPARENT) ;
      EndPath(ACanvas.Handle) ;
      ACanvas.TextOut(100, 500, TextToDraw) ;
    finally
      ReleaseDC(Handle, ACanvas.Handle) ;
      ACanvas.Free;
    end;
 end;

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procedure TFormScreen.Button1Click(Sender: TObject);
var
 DCanvas: TCanvas;
 DHandle: HWND;
begin
 DHandle:=GetDC(0);
if DHandle<>0 then
begin
    try
      DCanvas:=TCanvas.Create;
      try
        DCanvas.Handle:=DHandle;
        DCanvas.Font.Color:=clRed;
        DCanvas.Font.Size:=72;
        DCanvas.Brush.Style:=bsClear;
        DCanvas.TextOut(300, 100, 'Ein Schriftzug');
      finally
        DCanvas.Free;
      end;
     finally
       if ReleaseDc(0, DHandle)<>1 then
         RaiseLastOSError;
     end;
end
else
    RaiseLastOSError;
end;

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RedrawWindow(0, nil, 0, RDW_UPDATENOW or RDW_ALLCHILDREN or RDW_INVALIDATE or RDW_ERASE or RDW_INTERNALPAINT);