I would download this and run it:
https://www.bitdefender.co.uk/solutions/free.html as its much more likely to catch things than defender
I would backup everything you want to keep and clean install as its only way to know you got rid of it for good
on another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
change boot order in BIOS so USB is first, hdd second
boot from installer
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and reactivate once finished
On the screen where you choose where to install win 10, if it gives you an error about GPT drives, delete all the partitions on the hdd and press next. If it still gives error, cancel out of the installer and restart PC and start installer again, it will accept next on that screen this time (some PC just need a restart here)
install bitdefender free again and scan any of the files you backed up to ensure you don't get it back.