I am baffled! virus signature updates cannot happen in the background and instead, every few hours, I get a notification from Windows update that there are definition updates! how come half the world use this crappy AV and it can't even update signatures silently? I tried it at work also on my Win 7 computer (so it was MSE here) and same exact thing
As I recall it's the same thing on Windows 8.0. So I've set up a scheduled task to auto-update Defender once every hour, to avoid WU to bug me...
It sounds like you turned off auto-updates. In Windows Update, it has to be set to "Install updates automatically (recommended)" in order to receive silent virus signature updates (since it's through Windows Update).