After removing AVG 2015 Windows Defender will not start

System: Windows Pro 8.1

After removal of AVG 2015, Windows Defender fails to start.

When attempting to start Windows Defender via the UI

It fails to start with error 0x8007139f

When attempting to start the service

It fails with error :

Error 577: The Windows Defender Service service failed to start due to the following error: Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.

Does anyone know how to repair Windows Defender or otherwise get the service running again?

I finally figured out the fix to the issue. I had tried multiple registry edits suggested on many other forums. I had tried the System File Checker but that had always reported everything was ok. I had even re-installed AVG and used their removal tool. Still nothing got Defender re-enabled.

What actually ended up leading me to a fix was someone pointed me to an article where a user had uninstalled AVG and wasn't able to run Defender just like myself. For them the fix ended up being to remove SpyBot's active service. That seemed pretty reasonable but I didn't have SpyBot installed. It did get me to think of other programs that had a security focus though. I had Comodo's personal firewall installed. Comodo does offer other services such as AV in their suite of products but I was running just the firewall portion. Still, I figured it was worth a try. I uninstalled Comodo's software and that did the trick. I even re-installed the Comodo firewall and Defender is still running just fine.

So, the final fix was, uninstalling then reinstalling Comodo's firewall product.