After upgrading to latest version of Internet Security I hit a problem with Windows Update attempting (and failing) to update Windows Defender.
This didn't happen on all my PCs - only on those that had started life as Windows 7 and upgraded via W8 to W10.
Searching various Microsoft Sites I found a Registry hack that fixed it see below
Windows Registry Editor Version 5.00
; Created by: Shawn Brink
; Tutorial: http://www.tenforums.com/tutorials/5918-windows-defender-turn-off-windows-10-a.html
It appears that Security Centre doesn't fully disable Defender at start-up.