Malware Prevention Diagnostic Tool: An entirely different kind of scanner. Instead of looking for malware, it checks how you've configured Windows and offers to fix settings that can be exploited by the bad guys.
Windows Defender Offline: Another bootable scanner. Microsoft Standalone System Sweeper's replacement.
Windows Defender--the Windows 8 version: Like the two Windows before it, version 8 comes with Windows Defender. But this time, it uses the same antimalware engine as Security Essentials. In other words, it's a real antivirus program, but probably not one of the most effective.