Emma: I beleive the proper procedure would be:

  1. Down load avast! to desktop (or disc).
  2. Go offline
  3. uninstall All Norton and its components (other threads say how)
  4. install avast!
  5. go online and update avast!

If you had the Norton Suite then you also need to add Ad-AwareSE (or similar), Spybot S&D (or similar) and a firewall like ZoneAlarm (or similar) before you go back online. Other threads (especially the General and off-topic) cover these issues. Welcome back to the avast! fold.