procedure TForm1.Button1Click(Sender: TObject);
var
  DC: HDC;
  Canva: TCanvas;
  B: TBitmap;
begin
  Canva := TCanvas.Create;
  B := TBitmap.Create;
  DC := GetDC(0);
  try
    Canva.Handle := DC;
    with Screen do
    begin
      B.Width := Width;
      B.Height := Height;
      B.Canvas.CopyRect(Rect(0, 0, Width, Height),
      Canva, Rect(0, 0, Width, Height));
      B.SaveToFile('c:\Мои документы\screentofile.bmp');
    end
  finally
    ReleaseDC(0, DC);
    B.Free;
    Canva.Free
  end
end;