Windows Defender for Windows 10, Windows 8.1, Windows RT 8.1, Windows 8, and Windows RT provides built-in protection against malware. You can't use Microsoft Security Essentials, but you don't need to—Windows Defender is already included and ready to go. But if you’re looking to protect a PC with an older version of Windows, you can use Microsoft Security Essentials to help guard against viruses, spyware, and other malicious software. It provides free* real-time protection for your home or small business PCs.
Microsoft Security Essentials supports Windows 7 and Windows Vista and is available in many locales and languages. If it is not available in your preferred language, you can install it in any of the other languages listed here, and it will still work to help protect your PC.