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procedure ScreenToFile(aFileName: string);  
var  
  Bitmap: TBitmap;  
  JPG : TJPEGImage;  
  Canvas: TCanvas;  
begin  
  Canvas := TCanvas.Create;  
  try  
    Canvas.Handle := GetWindowDC(0);  
    try  
      Bitmap := TBitmap.Create;  
      JPG := TJPEGImage.Create;  
      try  
        Bitmap.Width := Screen.DesktopWidth;  
        Bitmap.Height:= Screen.DesktopHeight;  
        Bitmap.PixelFormat := pfDevice;  

        BitBlt(Bitmap.Canvas.Handle, 0, 0, Pred(Screen.DesktopWidth), Pred(  
            Screen.DesktopHeight), Canvas.Handle, Screen.DesktopLeft,  
            Screen.DesktopTop, Bitmap.Canvas.CopyMode);  

        JPG.Assign(Bitmap);  
        JPG.SaveToFile(aFileName);  
      finally  
        Bitmap.Free;  
        JPG.Free;  
      end;  
    finally  
      ReleaseDC(0, Canvas.Handle);  
    end;  
  finally  
    Canvas.Free;  
  end;  
end;