Installed Norton Security on 3/16/2015 as an upgrade to N360.
Since then, and not prior, Windows Update includes definitions, patches and the like for Windows Defender.
WinDefender is not running, as far as I can tell. In Services, Windows Defender Service & Windows Defender Network Inspection Service are not running but are configured for manual start (not disabled). Even as Admin, I cannot change start setting in either services' profile UI. Nor do I see Windows Defender related *.EXE files loaded in the Task Manager views.
FYI - My N360 subscription had lapsed for 2 weeks before I was able to apply upgrade to NS. During this time, I made no manual changes to Norton or Windows security apps. Also, I do use MBAM, the free version that only runs when user initiates a scan. I have had MBAM installed to this PC and Windows image from date of purchase w/o problem.