Unable to Start Windows Defender Anti-Virus?.

My PC is running the 64bit version of Windows 10 Home Edition version 1903 build 18362.388 and I keep everything updated to the latest versions. The last two years or so I have used the paid for version of Kaspersky Internet Security 2020. I use a VPN for a couple of hours each day mainly for tv series not available in my local area. About a few weeks back I started getting a lot of problems while on the VPN qBittorrents and my browser would freeze each other up at times. After a bit of trial and error I found that the problem was Kaspersky as when I shut it down I had no problems at all.

This of course was not ideal as of course during this time I had no AV, just Windows Firewall and the VPN tunnel. Today while looking for solutions I came across an article saying that the Windows Defender AV had been improved recently so decided to completely uninstall Kaspersky and start using the Windows Defender AV instead. I uninstalled it via the "uninstall" option in the Control panel and deleted all the Kaspersky related folders using the show hidden folders option from the User Data folders. After a restart I attempted to start the Defender AV but kept getting the "Unexpected Error. Sorry, we encountered a problem, try again" message.

I have tried all the various options regarding regedit and even added a file giving me access to the gpedit set them as instructed but to no avail I still get the error message. In services.msc it says that both Defender options are "Disabled" but I have no access to restart or any other options as they are all greyed out. I came across a piece on shutting down and restating the Security Center but again have no way of doing it as it is all greyed out. The Firewall is working ok only the AV is not. I am thinking that perhaps a system reset may be the only way out of it but thought I would ask here if anyone had any ideas as all the ones I found do not work.

A couple of months back while Kaspersky was installed my PC upgraded from the Windows 18++ version to the latest one and perhaps something happened then?. Anyway any help/advice is sorely appreciated.

f14tomcat said:

My first reaction to your problem, considering the Kasperky and the steps you used to get rid of it, and the resulting Defender limbo state, is to do an In-place Repair of your machine. You won't lose any data or apps, it will re-do the Windows files, including Defender. I'm afraid, based on what you've stated, that it would take many hours to fight thru manual changes to fix, with a low chance of success. In-Place will take about 30-60 minutes depending on your machine.

As always, we urge everyone to have a backup of the system and their data prior to any repair work.

Tutorial for In-Place Repair Repair Install Windows 10 with an In-place Upgrade

I followed your advice and it all went off without a hitch. All my apps programs etc are still aboard and Windows Defender A/V is now running as well, so thanks again for the help.