My computer got infected by a virus (one that Windows Defender did not detect) last week. I ran a Norton Bootable Rescue CD I had made and that cleared out the virus for good. However, when I went to the Windows Defender Windows, it told me that I already had an antivirus running and thus was switched off. Believe me, I have no anti-virus running.
I realized that this was one of the after effects of the virus (or maybe it was still running) and thus I resetted my PC. After that I had no problems.
Now, I'm just curious, How exactly does Windows Defender know that an antivirus is running?
I have Windows 10 Version 1607 (AKA, The Anniversary Update).