Windows 10 is meant to disable windows defender when you install another AV (such as avast), but it needs to be off.
I'm unsure how the upgrade from win7 to win10 might have had an impact, given that win7 had windows defender but that wasn't a full blown AV. But the upgrade to win10 should have got that out and replaced with the win10 version of windows defender.
What version of win10 are you running ?
As far as I'm aware, the latest win10 version is 1607 build 14393.576. This assumes you aren't running any of the insider builds as some have experiences issues with these and 3rd party AVs.
I have only been using win10 for a few weeks so I'm not very clued up on it, hopefully one of the more experienced win10 users can jump in.