Are you the only person who has enough access to install things on that computer? Has anything been installed recently? Suppose that someone else had tried to install a video game or utility that offered to also install Avast and suppose they forgot to uncheck "Install Avast" before clicking "Next."
So it could be that it was an add-on that rode in on the installer for another program. Lots of programs used to offer Avast or Norton as add-ons. I remember...Java...Flash...torrent apps....players...etc. Maybe it came with something like that.
Could be but no one admits to installing anything recently.