I can see no sign of malware and Avast is no longer active on the computer
What malware did avast report ?
That’s the part I don’t know. My brother owns the computer, it’s not mine. I didn’t saw the issue 
Unfortunately am now at an impasse as there is nothing I can do
I used a lot of tools to try fix MBR, but no success
All that remains is a fresh install
Thanks for your help.