I am repeatedly having this message come up in the message center. Not daily, more like weekly or a little longer between appearances, and I find it strange.
I thought that Windows Defender, like most antivirus programs, was a "set it and forget it" kind of affair where about the only time you'd get any kind of notification was when there was trouble downloading definition updates or a major update to the program itself needed to be installed (which wouldn't happen with WD because that would occur as part of Windows Update).
If I click the message and do the scan everything comes up clean, so I'm not concerned that I have viruses or that the system is not being monitored, at least for the most part, but I have not experienced this sort of behavior on machines with Microsoft Security Essentials under Windows 7 nor Windows Defender under Windows 8.1.
Any theories about why this is and what, if anything, I should do about it?