Windows Defender will not activate

Perhaps a year ago, a friend paid someone to provide a new computer. It is some HP model that came with Windows 8.1 pre-installed. It possibly also came from HP with one of the big name (Norton, McAfee, or whoever) "security scan" application installed. A freeware antivirus application (Avast), along with a few other freeware items, were added before it was delivered to him. His data collection was copied onto the new hard drive and various programs he already owned (e.g. Word) were installed.

Today I offered to do the MS offered update to Windows 10 for him. Before beginning I uninstalled the security scan program and the Avast antivirus, wanting to just go with Windows Defender under Win10.

After considerable hurry up and wait, Windows 10 Home was in place and running. My last effort was to be to assure that Windows Defender was running. It wasn't; it isn't.

In Settings, on the Windows Defender tab, the controls are grayed out. At the very bottom is an active line saying something like
Use windows Defender
When I click on it I get a message box saying Windows Defender is inactivated. If I am using some other antivirus product, I should check its security settings. No other information is offered.

There is not another program that I can find. Programs and Features on the Control Panel lists nothing that could have antivirus or other security functions. I suspect that some registry setting from one of the two now uninstalled programs caused Windows Defender to inactivate. A registry value to that effect was carried forward into the Widows 10 installation.

Unfortunately I don't know how to go about finding this, nor what to do about it. Is there some turn-on-Defender override that is hidden outside the normal settings? How can I get Windows Defender to wake up and take notice?

Microsoft Security Essentials (not Windows Security Essentials) will not run on Windows 8, 8.1 or 10 as the built-in Windows Defender is Microsoft Security Essentials as far as capability is concerned.