Inno Setup script sample part 2

version.pas

function InitializeSetup(): Boolean;
var
  oldVersion: String;
  uninstaller: String;
  ErrorCode: Integer;
begin
  if RegKeyExists(HKEY_LOCAL_MACHINE,
    'SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{F768F6BA-F164-4599-BC26-DCCFC2F76855}_is1') then
  begin
    RegQueryStringValue(HKEY_LOCAL_MACHINE,
      'SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{F768F6BA-F164-4599-BC26-DCCFC2F76855}_is1',
      'DisplayVersion', oldVersion);
    if (CompareVersion(oldVersion, '6.0.0.1004') < 0) then
    begin
      if MsgBox('Version ' + oldVersion + ' of Code Beautifier Collection is already installed. Continue to use this old version?',
        mbConfirmation, MB_YESNO) = IDYES then
      begin
        Result := False;
      end
      else
      begin
          RegQueryStringValue(HKEY_LOCAL_MACHINE,
            'SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{F768F6BA-F164-4599-BC26-DCCFC2F76855}_is1',
            'UninstallString', uninstaller);
          ShellExec('runas', uninstaller, '/SILENT', '', SW_HIDE, ewWaitUntilTerminated, ErrorCode);
          Result := True;
      end;
    end
    else
    begin
      MsgBox('Version ' + oldVersion + ' of Code Beautifier Collection is already installed. This installer will exit.',
        mbInformation, MB_OK);
      Result := False;
    end;
  end
  else
  begin
    Result := True;
  end;
end;