After an hour or so of frustration and dozens of web searches, I finally got Windows Defender running after uninstalling Norton Internet Security. None of the web pages I read gave me the outright answer, and only one of dozens even provided a hint: make
sure that the Security Center service is running.
My Security Center service was running, but it occurred to me that Security Center may still think that Norton is my active anti-virus product. On a hunch I stopped and started Security Center. Within seconds a box popped up saying that Defender was out
of date, and to click on the box to update it. When I clicked on the box, Defender opened, and I was able to download and install the latest updates. When I went back to Services I found that Windows Defender is now running and set to start Automatically.
I am hugely disappointed that Microsoft has not been able to provide this relatively simple (and in hindsight, fairly logical) fix.
Hopefully this post will save someone a bit of time and frustration.
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