폼위에 Image 컴포넌트를 하나 올리세요~~ ^^

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procedure TForm1.Button1Click(Sender: TObject);
begin
  Image1.Width  := Screen.Width;
  Image1.Height := Screen.Height;

  BitBlt(Image1.Canvas.Handle, 0, 0, Screen.Width, Screen.Height, GetDC(0), 0, 0, SRCCOPY);
end;