It's easy enough to get rid of but what I've read
is that Google pays Avast to sneak in the Chrome installation. It's alluded to here. Somewhere I read it was one US buck per successful install but I cannot confirm that.
In any event, it's only happened to me when using the maintenance feature within Avast and not running from the downloaded file no matter where it comes from. What happens is that Avast pops up a screen at the end of the installation that it needs to reboot. Upon rebooting, the Chrome icon is on the desktop. One just goes to Add/Remove and can get rid of it. If you like Chrome, no big deal.