Windows Defender and Microsoft Security Essentials help protect your PC from malware and other threats in exactly the same way. You can't use Microsoft Security Essentials with Windows 8.1, and you can't use Windows Defender with Windows 7 or Windows Vista.
Windows Defender works on Windows 8.1. It's built into Windows and ready to work as soon as you turn your PC on.
Microsoft Security Essentials works on Windows 7 and Windows Vista. You can download it from the Microsoft Security Essentials website.
Before the release of Windows 8.1, 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 (in Windows 7 and Windows Vista) and Windows Defender (in Windows 8.1).
You can find out more about using our products on our security software help page.
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