Actually, Malwarebytes doesn't touch Windows Defender at all. This
behavior is actually due to the way that Microsoft designed Defender so
that whenever another security program registers as AV protection under
Action Center/Security Center, Windows Defender automatically turns
itself off to prevent any potential conflicts. That said, by default
whenever Malwarebytes is being installed, if Windows Defender (or
Microsoft Security Essentials on Windows 7/Vista/XP for that matter) is
detected as being enabled, Malwarebytes will default to not registering
with the Windows Action Center/Security Center so that Windows Defender
will remain active, even alongside the Premium version of Malwarebytes
3.
However, some users are experiencing an issue where, for some unknown
reason, Malwarebytes is still registering itself with the Windows Action
Center even when Defender is active on the system, causing Defender to
disable itself as Microsoft designed it to. That is what's causing this
issue and to my knowledge the team still doesn't know why, but they are
investigating it.
In the meantime, if you do experience this issue then you should be
able to resolve it simply by opening Malwarebytes and navigating to Settings>Application and selecting the option Never register Malwarebytes in the Windows Action Center under the Windows Action Center
section then restarting your system. That should cause Defender to
detect that no other program is registered with the Action Center so
that it re-enables itself automatically (again, this is how Microsoft
designed it so that when users removed their third party AVs they would
not be left without any active protection on their systems).
I have seen one instance reported so far where a registry policy was
configured to disable Defender, however Malwarebytes does not toggle
that setting, even when registering itself with the Windows Action
Center, so how that setting/policy got modified I do not know, but it is
possible that either Defender did so itself or a past or present PUP or
malware threat did so.
