If you don’t have another solution installed, Windows 8 will provide you protection with a significantly improved version of Windows Defender.
Improved protection for all types of malware. The improvements to Windows Defender will help protect you from all types of malware, including viruses, worms, bots and rootkits by using the complete set of malware signatures from the Microsoft Malware Protection Center, which Windows Update will deliver regularly along with the latest Microsoft antimalware engine. This expanded set of signatures is a significant improvement over previous versions, which only included signatures for spyware, adware, and potentially unwanted software.
In addition, Windows Defender will now provide you with real-time detection and protection from malware threats using a file system filter, and will interface with Windows secured boot, another new Window 8 protection feature.
When you use a PC that supports UEFI-based Secure Boot (defined in the UEFI 2.3.1 specification), Windows secured boot will help ensure that all firmware and firmware updates are secure, and that the entire Windows boot path up to the antimalware driver has not been tampered with. It does this by loading only properly signed and validated code in the boot path. This helps ensure that malicious code can’t load during boot or resume, and helps to protect you against boot sector and boot loader viruses, as well as bootkit and rootkit malware that try to load as drivers.