welcome to the the forum.
i suggest sens avast is detection the infection but can’t delete it do a boot scan and send any infection it finds to the chest instead of deleting it.
also do a scan with malwarebytes animalware. to be on the safe side.
http://www.filehippo.com/download_malwarebytes_anti_malware/
download install. update and do a scan don’t forget to remove what it finds. if asking for a reboot of your system let it do so.
a few step you could begin with.
good luck and let us know on the progress or if you need more support.