[Cesar82]

@mausschieber
This "CenterInsideControl" function no longer exists in Inno Setup 6...

A possible almost similar solution is:
Delete line in color red and insert this parts in blue color.

procedure AboutFormOnShow(Sender: TObject);
begin
  TSetupForm(Sender as TSetupForm).Left := WizardForm.Left + (( WizardForm.ClientWidth -  TSetupForm(Sender as TSetupForm).ClientWidth) div 2);
  TSetupForm(Sender as TSetupForm).Top :=  WizardForm.Top + ((WizardForm.ClientHeight - TSetupForm(Sender as TSetupForm).ClientHeight) div 2);
end;

procedure AboutButtonClick(Sender: TObject);
var
  AboutForm: TSetupForm;
  AboutOKButton: TNewButton;
  AboutLabel: TNewStaticText;
  LogoImage: TBitmapImage;
begin
  AboutForm := CreateCustomForm
  try
    AboutForm.ClientWidth := ScaleX(380);
    AboutForm.ClientHeight := ScaleY(280);
    AboutForm.Caption := SetupMessage(msgAboutSetupMenuItem);
    AboutForm.OnShow := @AboutFormOnShow;
    //AboutForm.CenterInsideControl(WizardForm, False);