All currently supported versions of Microsoft Windows have anti-malware included – even a Windows noob like myself knows this. Neither the internal nor any third-party anti-malware will detect everything and 100% prevent or clean infections with malware.
And… this doesn't have to be malicious.
"Just use antivirus" falls short.
Maybe disable all of the non-MS apps at startup? and try to see if they boot up. If they do, run a deep clean on avast and see if they are really malware
Disclaimer: My Windows knowledge is stuck in XP era... it might be a little outdated.