Pretty sure avast had nothing to do with what happened here. Prevention:

  • A key step for all users and clients is to come here to have questions answered or help provided.
  • Included in that help is a caution to never use avast phone help that extends to actual TeamViewer session(s), help at the phone center is supposed to be free as long as the issue is covered.
  • Addtionally, an adblocker installed will eliminate most, if not all, bogus avast or other support companies, in a Google search.

https://www.consumer.ftc.gov/articles/0346-tech-support-scams recommended reading for all clients.