@bill any Linux grandma should be using will check daily for updates for every single bit of software grandma is likely to install.
I assume Windows 8 does the same for software installed from the Store, finally bring it to parity with what Linux has had for about a decade (still no dependency management, I thin, right?).
I am not too worried about grandma type users. It is really unlikely that my wife, kids or father (they all use Linux) will end up with a infected machine. I am taking about additional security measures to cover potential weaknesses points such as web browsers, before someone finds an exploit.
The very fact that Windows Defender is (among other things?) a virus scanner tells me that Windows is nowhere near as secure as Linux. Linux does not need a virus scanner because there are no viruses. Linux security relies on architecture and configuration, Windows still relies on cleaning up the mess.
Despite my paranoid attitude to desktop security, the only things I have able to do find that Ubuntu does not do out of the box, is to restrict (at the OS level) which folders Firefox can access. I am advocating a configuration change to optimise out of the box settings for tight security.