Windows Defender is built into Windows 8. It helps protect and clean your PC from malware such as viruses, trojans, and worms. It also helps protect against potentially unwanted software, such as adware.
If your computer is running Windows 7 or earlier, you can download and use Microsoft Security Essentials.
You can’t use Microsoft Security Essentials and Windows Defender at the same time, but you don’t need to - they both protect your PC from malware and other threats in exactly the same way.
Before the release of Windows 8, Windows Defender referred to a separate tool that focused only on antispyware. This tool is no longer supported, as it is now fully integrated into Microsoft Security Essentials and Windows Defender.