Are you sure your installation of Windows Defender is working
correctly? You might be well advised to test Defender's detection of
the harmless EICAR test file to be sure:
http://www.eicar.org/anti_virus_test_file.htm
Note that you'll probably have to disable your installed real-time
Antivirus during the test because Antivirus programs also will pick up
the EICAR file, usually before Defender will detect it. People using
Defender should know what a WD detection looks like, and be sure their
protection is proactive.