procedure TFormMain.GridCheckBoxClick(Sender: TObject);
var cb: TCheckBox;
begin
    cb := Sender as TCheckBox;
    ShowMessage(IntToStr(cb.Tag));
end;

procedure TFormMain.clean_previus_buffer;
var NewCheckBox: TCheckBox;
    i: Integer;
begin
  for i := 0 to Grid.ColCount do begin
    NewCheckBox := (Grid.Objects[i, 1] as TCheckBox);
    if (NewCheckBox <> nil) then begin
      NewCheckBox.Visible := false;
      Grid.Objects[i, 1].Free;
    end;
  end;
end;

procedure TFormMain.set_checkbox_alignment;
var NewCheckBox: TCheckBox;
    Rect: TRect;
    i: Integer;
begin
  for i := 1 to Data.nQ do begin
    NewCheckBox := (Grid.Objects[i, 1] as TCheckBox);
    if (NewCheckBox <> nil) then begin
      Rect := Grid.CellRect(i, 1); // получаем размер ячейки для чекбокса
      NewCheckBox.Left := Grid.Left + Rect.Left + 5;
      NewCheckBox.Top := Grid.Top + Rect.Top + 5;
      NewCheckBox.Width := 14;
      NewCheckBox.Height := 14;
      NewCheckBox.Visible := True;
    end;
  end;
end;

procedure TFormMain.GridDrawCell(Sender: TObject; ACol, ARow: Integer;
  Rect: TRect; State: TGridDrawState);
begin
  if not (gdFixed in State) then
    set_checkbox_alignment;
end;

procedure TFormMain.AddCheckBox(i : Word);
var NewCheckBox: TCheckBox;
begin
  NewCheckBox := TCheckBox.Create(Application);
  NewCheckBox.Width := 0;
  NewCheckBox.Visible := False;
  NewCheckBox.Color := clWindow;
  NewCheckBox.Tag := i;
  NewCheckBox.OnClick := GridCheckBoxClick; // Связываем событие OnClick с существующим
  NewCheckBox.Parent := Panel2;
  Grid.Objects[i, 1] := NewCheckBox;
  NewCheckBox.Checked := True;
end;

procedure TFormMain.FillMatrixGrid;
var i: Word;
begin
  with Grid do begin
    ColCount := Data.nQ + 2;  // 2 = Имена и Всего
    RowCount := Data.nM + 3;  // 2 = Индексы, Чекбоксы и Верные

    ColWidths[0] := 161;
    Cells[0,0] := 'Коды';
    ColWidths[ColCount-1] := 50;
    Cells[ColCount-1, 0] := 'Всего';
    clean_previus_buffer; // очищаем старые чекбоксы...
    for i := 1 to Data.nQ do begin
      Cells[i,0] := IntToStr(i);
      AddCheckBox(i);
    end;
    set_checkbox_alignment; // расположение чекбоксов в ячейках таблицы...
  end;
end;