Updated 13 Jan 2015 - The apparent solution I posted was wrong so I have deleted it and replaced it with the fix that does work.
Comments:
I had used the tool provided in How to temporarily prevent a Windows or driver update from reinstalling in Windows 10 to hide KB2267602 but this was inadequate. The tool only hid the current version of the KB so when the KB with updated definitions was later released, Windows update saw it as a new update and the whole download, install, fail process repeated itself.
I contacted Norton support and, after a lot of pointless toing & froing {they must be paid by the hour}, they stopped denying that it was a shortcoming in Norton that produced this strange disabled-yet-still-updating state of Windows Defender. I asked for a full refund of my subscription and got it.
So now I have reverted to Windows Defender on my Windows 10 computer and have reinstalled MSSE on my Windows 7. All in all, this has cost me two weeks of my limited data allowance in the space of a couple of days. After I had removed Norton, I stumbled across a claimed solution that actually works:
The fix that does work
In Solved Defender and KB2267602 update failure - Windows 10 Forums, a Norton user reported multiple symptoms including the one that is the subject of this thread {it is not worded very clearly so you might need to read it a few times to realise this}. The solution is to put in a Registry key that completely disables Defender and puts an end to these failed updates - see Windows Defender - Turn On or Off in Windows 10 - Windows 10 Forums. The user did not give any link to the solution so I searched for it and the link I have given matches the title & the author's name given by that Norton user and it is clearly aimed at the problem users in this thread have been experiencing.
As you'll see on the next page of this thread, Jeffrey has since applied this fix and it works.
He ran the Norton Removal Tool,
applied the fix to completely turn off Windows Defender - Windows Defender - Turn On or Off in Windows 10 - Windows 10 Forums
then re-installed Norton.
I suspect that the problem is the result of an undetected Norton installation error. That would explain why some users have not experienced this problem and why others have reported that the problem was cured by reinstalling Norton [in some cases after manually disabling all Windows Defender entries in Settings, Update & security, Windows Defender before even starting the reinstallation].
Best of luck,