If you already have an Ask toolbar installed on your computer, the Avira toolbar and the Avira Web Protection cannot be installed.
Hence, there is no corresponding installation option shown for the Avira toolbar during the installation of Avira Free Antivirus.

Via Start → Control Panel → Programs and Features (in XP: Start → Settings → Control Panel → Add or remove programs) you can tell if the Ask.com-toolbar has already been installed.

If you still want to install Avira Toolbar, we would like to refer you to the following article that explains the removal of the Ask.com toolbar and the installation of the Avira toolbar: The Avira toolbar does not appear in the setup and the browser when the Ask.com toolbar has already been installed