Hi Halo, Charles, and others,
As you can tell, you are not alone with this problem. Since I had a similar incident
one time, that was caused by a service that was not running, that is easy to check
and get out of the way. Go to "services", and scroll down to "Security Center".
Verify that it is running and is set for Automatic (delayed start). Scroll on down
the list to Windows Defender. All four of them should be running as well.
I have assumed before, that anyone with this problem, has thoroughly scanned
for malware. In case you haven't, the BitDefender Rescue Disk might be something
you want to try. Here are two links to it, for preparation and use. Suggestion, download
the Stickifier Executable" and just run it from your download folder. Use a USB.
How to Create: http://www.bitdefender.com/support/how-create-a-bitdefender-rescue-cd-627.html
How to use: https://www.bitdefender.com/support/how-to-scan-your-computer-with-bitdefender-rescue-cd-1250.html
If you are satisfied that no malware can be found, and you still have your problem,
then perhaps a Repair Upgrade is in order. It replaces your Operation System,
including Windows Defender, but leaves your Apps and Files intact. It sounds like
a much bigger task than it really is.
Just to be secure, I recommend that you create a full Recovery Drive USB first.
In a nutshell, here is all that it amounts to, with a few normal decisions from you.
Download the Media Creation Tool from here: I am writing this from a W7 PC,
and Microsoft has apparently seen fit to disallow me to enter this link on it !!!
https://www.Microsoft.com/en-us/software-download/windows 10
Once the tool is downloaded, find it in your download folder, right-click
and select "run as administrator". Note: when you are asked to allow updates
( the recommended option) Decline (for now). Get them later from Windows
Update. A more secure way to do it. Keeps update sequencing better.
Select, Run as for another computer. Now you control when the repair starts.
Select the download to be ISO. The most efficient way to do the repair.
The ISO ends up in your Documents folder. On fast internet, about 30 minutes.
Right-click the ISO and select "mount".
Double-click "setup.exe" And the repair starts. When you are asked what to keep
Ensure that both items are checked. This keeps your Apps and Files intact.
The repair takes about an hour and a half. Your computer will restart several times.
Be patient, and don't interrupt it. After it is finished you will see your "sign in" screen.
Now go to Windows Update and "check for updates". Once they are installed, you
can try to access Windows Defender again.
It is brand new, so it should be working. If it is not, your problem has to be with
one of your Apps, or malware disguised as an App. If you should end up in this
situation, perform a Clean Boot, to discover which one.
PS; If your problem is fixed, re-make your Recovery Drive USB. It contains your problem.
Good Luck, Glen