Avast is not a virus, it is a very good anti virus, but I agree with you nothing should be secreted onto your PC without your agreement, it sounds like the update you are using has added Avast to your PC may be as a free trial or as usually its called a PUP.
I have used Avast (Free version) for more than 12 yrs, unfortunately, the company was sold and now most disreputably the popups have started and they are a flaming nuisance, if you actually pay for the program there are about 7 other extras you could also pay for or you will get the dreaded popups.
My point is the internet was made for the people, and not to be commandeered by the likes of M'soft Google and Amazon so the regulators whoever they are should make international law make secreting anything onto your PC from a nasty virus to a free trial PUP illegal, with very high penalties to suit