Disabled Windows 10 Defender.

My Windows 7 computer is running Avast Free Version 11.1.2241without problems. Windows 7 Defender is on and protects this machine from a particular virus that pops up on my wife’s machine when she is on “https://www.zinio.com” attempting to read a magazine. Her machine was a Windows 7 machine but I upgraded it to W10 about a month ago.
The only way to get her machine to bounce the virus is with Windows Defender, but with Avast or AVG Free installed, both anti virus programs disable Defender and I have not been successful in turning it back on. I have tried going into the registry as an admin, but when I attempt to change it, regedit says that it is protected. I have even attempted to run a .reg file to do it. Neither will turn on Defender. It acts like the entry is locked and there is no way to unlock it. The only way to turn Defender on is to totally remove Avast.

What can be done to solve this problem.

What is telling you that it isn’t a false positive from Windows Defender ?

Report the “infection” to avast.
If it is indeed malicious they will add detection for it.
https://support.avast.com

https://blog.kaspersky.com/multiple-antivirus-programs-bad-idea/2670/