From the looks of things you might need to re-install it, disable it's Self-Defence mode, then you can uninstall it.

How to Uninstall Avast Antivirus

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Grant
Darwin NT

Luckily these days installing a third party antivirus is not needed, the one Windows already has is effective enough, if you regularly update Windows.

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Luckily these days installing a third party antivirus is not needed, the one Windows already has is effective enough, if you regularly update Windows.

Yes,its a solution.
Even the windows 7 security/antivirus package was probably efficient and enough.

But, since the support for w7 stopped at some point of this year even tough avast announced that the support for this os w7 will last I installed avast.
Then this happened.
"Antivirus software searching for a cure" etc.

And also, comparing to the behavior of other software on which you can have some sort of administrative control "avast free" is possibly dangerous and a backdoor opener.
Still, not a reccomendation for use. The way to get rid of it, completely uninstall is using youruninstaller pro , deleting registry keys, restarting and then if the directory stays in installation folder (program files) delete for eg. with different bootable OS.

Free AVG works for me but has ALOT of nagware and requires a reboot about once a month but it does work for my Boinc only machines still running Win7.
Even the free version of Malwarebytes has nagware now!!
BUT I have 15 pc's here and I am not paying for that stuff on a machine that only crunches!!
I do use the free Win10 stuff on my Win10 machines except for my laptop I use all the time, my desktop I use when I'm not using the laptop and my backup machine that all the other pc's do a monthly backup to.

Always hated Avast, it just recently acquired CCleaner and straight away we were all getting infected with viruses, literally told us which files to delete in our Registry claiming that would be the only time we'd get infected but i knew that was a lie. As long as they've been around i've considered them just Malware NOT for cleaning any viruses because they then infect you with viruses and their software behaves like one... and it's been this way from the very start in 1995 when their company first came into existence it's dodgy and i've no idea how they're aloud to continue.

We all tried Avast after 1995 and i remember clicking through it and my dad had to reinstall the Operating System because it infected the computer with a lot of viruses, i have a feeling it's just a Data Mining company because around that time there was the Bonzi Buddy thing which was also considered a Data Miner, how Avast is aloud to keep mining data i've no clue any other company would get shut down for doing this.