The antivirus program that comes with Windows, starting with Windows 8 (it was retroactively added to Windows 7). Windows Defender scans files downloaded from any Windows Web browser. Defender -> MSE -> Defender Windows Defender was originally only an antispyware program that was acquired by Microsoft from GIANT Company Software in 2004. Defender was replaced by Microsoft Security Essentials (MSE) in 2009, which included antivirus scanning. In 2012, the name was resurrected for the antivirus program built into Windows 8, and it uses the same virus signatures as MSE. See Microsoft Security Essentials.