Microsoft Security Essentials is a new, no-cost, anti-malware solution for genuine Windows PC consumers that provides real-time protection against viruses, spyware and other malicious threats. It is a lightweight, effective and modern anti-malware which runs on 32 bit and 64 bit Windows 7, Windows Vista and Windows XP SP2 and higher, and on modern consumer form-factors such as netbooks.
A beta version of Microsoft Security Essentials v1.0 is available today for up to 75,000 consumers in a limited number of countries. You can find more details about the beta at the Microsoft Security Essentials website.
— Microsoft Security Essentials Team