procedure TForm1.Button1Click(Sender: TObject);
var R : TRect;
begin
    SystemParametersInfo(SPI_GETWORKAREA, 0, @r, 0);
    Label1.Caption := IntToStr(r.Top);
    Label2.Caption := IntToStr(r.Left);
    Label3.Caption := IntToStr(r.Bottom);
    Label4.Caption := IntToStr(r.Right);
end;