This is easy to solve. You have the beta version of Defender and it expired in December. You need to remove it and install the latest version.
Remove Windows Defender:-
Start - Control Panel - Add/Remove Programmes
Highlight Windows Defender, and click Remove.
I recommend restarting the computer when you've done this.
Now download the latest Defender here:
http://www.microsoft.com/downloads/details.asp x?********=435BFCE7-DA2B-4A6A-AFA4-F7F14E605A0 D&displaylang=en
There have been lots of complaints about this, by the way, so you're not alone.