Enable Windows Defender after uninstalling Avast antivirus (Error 577)

Avast seems a nice antivirus but it can also become really annoying with all those ads that emphasize supposed difficiencies to promote the program’s commercial version, but also with too many false alerts. Sometimes you may face difficulties even to terminate a false alert and add an exclusion!

After uninstalling Avast I thought that Windows Defender would be automatically activated or at least popup a warning asking me to activate it. Nothing happened! It seems that Avast intentionally or not didn’t change some of its configuration that may have disabled certain Defender functions.

It was not possible even to start the antivirus service of Windows Defender from its main interface, so I tried the Services panel in Windows Settings. Defender Services refused to start, giving a message that, “Windows could not start the Windows Defender Antivirus Service service on Local Computer. Error 577: 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.”

I solved this problem and was able to activate completely all functions of Windows Defender from its normal main interface, only after I changed a registry setting. At \HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender there is this key named “DisableAntiSpyware”. Edit this key to change its value from 1 to 0, and enjoy!