[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program
OutputDir=userdocs:Inno Setup Examples Output
VersionInfoVersion=19.81.9.81

[Code]
procedure InitializeWizard;
var
  Version: string;
begin
  // check if there's any previous directory; if so try to get the version string; if that succeed,
  // check the current VersionInfoVersion value is lower than the obtained version; if that's true,
  // show the message and exit setup by throwing a silent EAbort exception
  if (WizardForm.PrevAppDir <> '') and
    GetVersionNumbersString(AddBackslash(WizardForm.PrevAppDir) + 'bin\application.exe', Version) and
    (CompareText('{#emit SetupSetting("VersionInfoVersion")}', Version) < 0) then
  begin
    MsgBox('You''re going to install older version! Setup will now exit...', mbInformation, MB_OK);
    Abort;
  end;
end;