[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program
OutputDir=userdocs:Inno Setup Examples Output

[Components]
Name: "file1"; Description: "File 1"
Name: "file2"; Description: "File 2"
Name: "file3"; Description: "File 3"

[Code]
procedure ComponentsClickCheck(Sender: TObject);
var
  Item1: Integer;
  Item2: Integer;
  Item3: Integer;
  ListBox: TNewCheckListBox;
begin
  ListBox := TNewCheckListBox(Sender);
  Item1 := ListBox.Items.IndexOf('File 1');
  Item2 := ListBox.Items.IndexOf('File 2');
  Item3 := ListBox.Items.IndexOf('File 3');

  if (Item1 <> -1) and (Item2 <> -1) and (Item3 <> -1) then
  begin
    if (ListBox.ItemIndex >= 0) and (ListBox.ItemIndex <= 1) then
      ListBox.Checked[Item3] := ListBox.Checked[Item1] or ListBox.Checked[Item2]
    else
    begin
      ListBox.Checked[Item1] := ListBox.Checked[Item3];
      ListBox.Checked[Item2] := ListBox.Checked[Item3];
    end;
  end;
end;

procedure InitializeWizard;
begin
  WizardForm.ComponentsList.OnClickCheck := @ComponentsClickCheck;
end;