If you start running into the FBI scam, Switch to a chromebook. The FBI file comes up initially but the files it is looking for are not there. Specifically it seems to focus on Norton files. Norton does not run on a Chromebook. When you turn off the Chromebook, the FBI message does not return.
Over many years of supporting computers, DOS and Safemode can work to delete a stubborn virus It takes awhile to find the files but it is doable. If you go onto CMD, DOS is not affected by viruses and the manual commands work fine. If you know the name and address of a virus file, it can be slowly navigated to than deleted in DOS. Usually, a back up file is also infecting the computer, its job is to replicate the deleted file when you restart the computer. Usually 2 or 3 files need to be deleted to remove a well design virus to avoid file regeneration after they are deleted.
These are low tech solutions but they work if you know DOS and Safemode. If you are not comfortable with them than do not try anything.