If you are using Google Chrome as your default web browser, you will be unable to use Avira Web Protection since the Avira toolbar (or Ask.com toolbar respectively) can only run on Internet Explorer 6 or higher and Firefox.

Please consult this page for more details:
http://about.ask.com/apn/toolbar/docs/default/faq/en/ie/index.html#na2

Additional note:

If you are using Opera as your default browser, the Avira Toolbar will still be installed, but only in the Internet Explorer and Firefox.