Windows Defender Offline is a standalone software application that is designed to help detect malicious and other potentially unwanted software, including rootkits that try to install themselves on a PC. Once on a PC, this software might run immediately, or it might run at unexpected times. Windows Defender Offline works by scanning an operating system to check the authenticity of any communication the operating system has with the Internet. If there is an application deemed unsafe, it will alert the user and block the contents of the application until the user either accepts or denies the risk. Learn more.