Windows Defender - Turn On or Off in Windows 8. Windows 8 comes with a new version of Windows Defender which is the re-branded Microsoft Security Essentials (MSE). When Windows Defender is on, you're notified when viruses, spyware, malware (malicious software), or potentially unwanted software try to install or run on your PC. This tutorial will show you how to turn on or off Windows Defender in Windows 8, Windows RT, Windows 8.1, and Windows RT 8.1.