The same thing happened on my computer yesterday and I closed the window down.
It reappears each time after rebooting my computer.
It's not a typical Microsoft update notification so I've attached a screen shot

I'm wondering if this is the same update as KB2687502
The August 12, 2014 update for Office 2010 Spelling and Grammar Check feature?
If so it's sure an odd way for MS to push an update via a popup as opposed to including it with Tuesdays "Windows updates".
If it is indeed the MS update I would appreciate a confirmation from someone @ Microsoft.