RECOMMENDED: Windows Defender is the default antivirus software in Windows 10. The Defender program helps protect your Windows operating system as well as data from common threats like viruses, spyware, and malware. With default settings, Windows Defender automatically downloads antivirus definitions from Microsoft servers when new updates are available. Usually Microsoft releases new definitions every day through Windows Update service. So it’s important to keep the Windows Update turned on in order to automatically update Windows Defender.