Call the Windows API function SystemParametersInfo() passing the SPI_GETWORKAREA parameter along with the address of the rectangle structure to receive the coordinates. Example:

procedure TForm .Button Click(Sender: TObject);
var
  r : TRect;
begin
  SystemParametersInfo(SPI_GETWORKAREA,
                       0,
                       @r,
                       0);
  Memo .Lines.Add(IntToStr(r.Top));
  Memo .Lines.Add(IntToStr(r.Left));
  Memo .Lines.Add(IntToStr(r.Bottom));
  Memo .Lines.Add(IntToStr(r.Right));
end;