Are you using CCleaner? Piriform has programmed it for some reason to remove part of the MSE History files and it's deleting the files MSE checks to determine the last time a scan was conducted. This file removal has no effect on the antimalware protection provided by MSE.


Open CCleaner Applications Tab, then Under Windows uncheck MS Antimalware...that should prevent CCleaner from deleting the MS Antimalware scanning history files.

You may also need to uncheck MS Management Console.


You may need to run another quick scan and reboot the computer.