Hi Aira. I'm Greg, an installation specialist, 9 year Windows MVP and Independent Advisor, here to help you.
Try running Avast Clear from Safe Mode with Networking, or Safe Mode accessed by one of these methods: https://www.digitalcitizen.life/4-ways-boot-saf...
Avast Clear here: http://www.avast.com/en-us/uninstall-utility
Then make sure Defender turns on in Security Center:
http://www.thewindowsclub.com/unable-to-turn-on...
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.
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