Hi britechguy. Thank you for sharing. Good to know it's not just me. Sorry you are having the same problem. I didn't have any anti virus at first as I heard that Windows Defender comes built in and was supposed to be all we needed with Windows 10, but since it won't turn on, I have now downloaded and am using AVG Free anti virus. I've used it for years and have always been happy with it.
Cheryl, if you're happy with AVG, then you don't need Windows Defender & that's why it doesn't turn on.
If you were running Windows Defender on Windows 7 prior to running Windows 10, that's a different software, more of a anti-spyware software. Windows Defender for Windows 8 through 10 is kind of what MSE is to Windows 7 & Vista, a full fledged AV software. You don't want both Windows Defender & AVG running at the same time. Not only will your PC use a lot more resources, both running at once will lessen your protection, not make it better, because both would be bumping heads.
So yes it's built in, yet if you have any other AV software installed, it'll shutdown.
Hopefully this helps to explain
And whatever you do, please do not use the registry cleaner of ASC on your computer. These type of programs can cause a lot more harm than good. As far as IObit Malware Fighter goes, I just don't trust the brand. It's been sometime back, they were caught red handed stealing technology from Malwarebytes, and claiming the software as their own. The reason they were caught was that the Malwarebytes team set them up by inserting a couple of key files in the software, once a thief, always one. I wouldn't install anything with the IObit name on it if given to me, even a Lifetime subscription. Yet that's me, but I bet there's others who feels the same way.
Bleeping Computer does not endorse the use of registry cleaners, while there may be a few keys no longer needed, these poses no harm. The only time that registry keys should be quarantined or deleted is if a reliable anti-malware (AM) software, such as Malwarebytes Anti Malware or Emsisoft Anti Malware (the latter is a AV+AM in one). Furthermore, the registry doesn't need to be defragged, this is snake oil, in other words, useless, as soon as that ASC license expires, it needs to be removed. If you have any auto scans to be ran to clean things up, you need to disable these, the only things that really needs cleaning are your browsers & Temp files, and unless just performing a transaction, doesn't need to be ran daily.
While AVG has had it's ups & downs, overall it's a good choice for Windows 10, in fact when running the first Previews late last year, posted some screenshots that showed AVG catching one of the keyloggers, at least one member felt that I shouldn't have done that.
I thought that to be great, as even the Emsisoft Emergency Kit (EEK) didn't catch the keyloggers. As long as you're running AVG, you're better protected than running Windows Defender, I've yet to see it catch anything.
Good Luck with Windows 10!