I recently underwent an exercise in frustration – taking a stock Lenovo G50 laptop and trying to get it into a workable state. Like many lower end laptops it came with a lot of bloatware, not the least of which was Mcafee Antivirus-premium-whatever garbage.
When I removed Mcafee and rebooted, I got a notice that Windows Defender was not enabled. When I clicked to enable it… nothing happened. I tried several times to no avail. I tried manually starting the service and it failed. I tried hard core things like running sfc /scannow and even editing the registry per here, but the issue remained.
Finally I came across this article which showed me how to fix it – simply launch windows defender by hitting start and searching for Windows Defender. Click the big red “Turn On” button. That’s all it takes! Why you can’t do that through the action center or by manually starting services is beyond me. Amazing.
Note: it’s possible that the combination of editing the registry, rebooting, and then launching the Windows Defender program fixed the issue. Whatever – it works now 🙂