system
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I’d scan with at least one Rescue Disk and completely format the drive in a new installation.
You might go look for yourself some AV-rescue disks or you could use SARDU, but be prepared with “virus/PUP alerts” depending on your heuristic/PUP settings, some adware to opt-out/decline if you don’t want them and to temporarily disable AV-realtime protection during the creation of the USB-stick (last time I had to do that with Avast; don’t know about nowadays, though).
Make sure to include external drives in the scan just in case some malware has been backed up there, too.
After you’ve cleaned everything with AV-rescue tools so far and backed up only the most necessary data, boot from your Windows DVD and use the “clean all” command in diskpart to completely wipe your whole boot drive.
If you’re finished, install Windows+needed drivers+updates+programs like Avast and then be sure to make an image backup, for example with Macrium Reflect or with Paragon Backup and Recovery.
You might also make a system image backup right after installing and applying Windows updates and drivers to have a completely clean system you can restore in case you screw up right at the installations of programs or if you want to have an image without any AV so that you can cleanly install and try any security software you want without having remnants.
After that, scan the backed up data with Malwarebytes and/or Avast at least once and only after that you should copy the data back.
If everything comes out clean again, make sure to include another image backup.