Hello Phil,
I recommend creating a Windows 11 USB install, tap or click here for the download.
Then, I would reinstall Windows 11 from the local USB drive while ensuring your machine is not hardwired via Ethernet.
When the machine prompts you to connect to the web, do the following.
When the first dialog box appears during setup, hit Shift + F10 and type OOBE\BYPASSNRO into the command prompt shell that appears. This will disable the internet connection requirement and force a reboot of the machine to restart the setup process.
I have tested this with the latest Windows 11 Media creation tool as of today.
Once logged in. If you click the small carrot near the time in the lower right corner, you can find the two-tone blue shield called “Windows Security.”
Upon checking out the settings, everything, including virus & threat protection, is running fine. Even after installing updates.
Looking more into it, with Windows 11 they no longer call it Windows Defender, Windows security now.