Thanks everyone & I'll check out Unchecky.
I can understand if I accidentally failed to uncheck a box because that has happened. But in this case I was never presented with anything to uncheck.
One of the issues is that the pre-checked opt-in box could be in one of two possible locations. Depends I guess on how you install Avast.
If installing Avast for the first time (or a clean reinstall) it could appear on one of the first installation windows, before you actually get into the Avast installation. I haven't seen this there for some time now, so I don't know if they have changed that.
Where I have seen it recently is after doing a manual program update, you need to do a reboot for the update/installation to take effect. Then you are likely to get a popup window, if you happen to miss it or are in a hurry close that window or leave that window and you are likely to get Chrome.