HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend Set "Start" to "4" and reboot. The service should no longer start and won't update.
We already tried that with cmd man using the automated means to set service using reg add. It is much more efficient and means we don't have to do registry diving.
It's a little unclear, but it sounds like that doesn't work or requires additional permissions. Manually going to the registry key didn't require anything other than running regedit as an administrator.
Another thing you could try if only needing access to one specific reg file is right click on the folder needed in registry which holds the key you wish to change and goto permissions, see if everyone is listed...if so then make sure all boxes are checked and save it. If not seeing everyone listed then add it and when created check all boxes and save. You should then be able to change what you need. When finished editing the registry remove or change permissions for everyone and you are done. It might work but if your permissions are messed up to begin with then you need to fix that first.