Microsoft recently contacted few selected Windows 7 test group for the upcoming release of Microsoft Security Essentials (MSE) anti-malware product, and on Oct 24th, made the new test build 1.0.1676.0 available via their Connect site for Windows XP 32-bit (8.62MB), Windows Vista/7 32-bit (4.29MB), and Windows Vista/7 64-bit (4.72MB). Ars said that every installer grew by 0.01MB with this new build. The notable changes among others include: “integration within Outlook and Windows Live Mail, and memory improvement in MsMpEng.exe”.
Update: To participate in on going beta program, go to Microsoft Connect using following link and login with your Live ID. It'll register you and will get you download links.