It is now out of Beta and officially called Windows Defender Offline.
Sometimes, malicious and other potentially unwanted software, including rootkits, try to install themselves on your PC. This can happen when you connect to the Internet or install some programs from a CD, DVD, or other media. Once on your PC, this software might run immediately, or it might run at unexpected times. Windows Defender Offline can help remove such hard to find malicious and potentially unwanted programs using definitions that recognize threats. Definitions are files that provide an encyclopedia of potential software threats. Because new threats appear daily, it's important to always have the most up-to-date definitions installed in Windows Defender Offline. Armed with definition files, Windows Defender Offline can detect malicious and potentially unwanted software, and then notify you of the risks.