Secure Boot
Microsoft designed Secure Boot to protect the computer from low-level exploits and rootkits and bootloaders. A security process shared between the operating system and Unified Extensible Firmware Interface (UEFI, replacing the BIOS), Secure Boot requires all the applications that are running during the booting process to be pre-signed with valid digital certificates. This way, the system knows all the files being loaded before Windows 8 loads and gets to the login screen have not been tampered with.
If a bootloader has infected your computer and it tries to load during the boot-up sequence, Secure Boot will be able to undo all the changes and thwart the attack. Having Secure Boot means it is that much harder for attackers to try to compromise the start up sequence.
While PC makers have to have Secure Boot enabled in the UEFI firmware by default, if they want to be able to slap the Windows logo outside the box, the feature can be disabled within the UEFI interface. Anyone who wants to install a non-Windows operating system on Windows 8-certified hardware would first have to manually disable SecureBoot.
Loading the AV First
Regardless of whether you are using Windows Defender or a different anti-malware product, Windows 8 has tweaked its load process so that security software runs first. Early Launch Anti-Malware (ELAM) insures that the first software driver loaded into Windows 8 is a driver from the user's anti-malware software.
In previous versions, if the malware executed and was loaded into system memory before the operating system and the antivirus, it was difficult to detect and remove. SecureBoot prevents rootkits from interfering with the OS, and ELAM ensures that pre-approved anti-malware software drivers are loaded before any other application.
SmartScreen
Originally an Internet Explorer security feature, Microsoft added SmartScreen to Windows 8. When a user downloads a program or a file from the Internet, the SmartScreen filter checks to see if other people have downloaded the same file as well. If so, there is a rating for the file based on its popularity and whether it was considered malicious. Users trying to download something with a low rating while Smart Screen is enabled will see a warning message. This can be good for detecting fake antivirus and other rogueware programs.
Since SmartScreen is now part of Windows 8, the filter will kick in regardless of what browser the user is running, not just Internet Explorer.