As a Microsoft Security Essentials Prerelease user, you will have the opportunity to explore and test new builds of Microsoft Security Essentials before they are publically available and provide feedback to Microsoft. Your feedback helps Microsoft to make its software and services the best that they can be. As a Microsoft Security Essentials Prerelease user, Microsoft Security Essentials updates will automatically be installed on your computer through Microsoft Updates.
To help improve the software, you will be enrolled in the Microsoft Error Reporting, Customer Experience Improvement Program, and Microsoft Active Protection Service. To learn more about these programs, see the Microsoft Security Essentials privacy statement at http://go.microsoft.com/fwlink/?linkid=195299 .
Leaving the program: If you wish to leave the prerelease program at any point in time, you can do so by uninstalling the program from your computer.
If you wish to be able to submit feedback to Microsoft about this program you have to register to the Security Essentials Prerelease program on Microsoft Connect: http://go.microsoft.com/fwlink/?LinkId=254220.
Important: If you are running any version of Security Essentials, you have to uninstall first before installing the Pre-release version of Security Essentials.
You must be running a Genuine Windows to install Microsoft Security Essentials. Supports Windows XP Service Pack 3 (SP3), Windows Vista (Service Pack 1, or Service Pack 2), Windows 7
I have been running & testing this Prerelease since couple of months and now its been made available publically too.
You can download it from here: http://www.microsoft.com/en-us/download/details.aspx?id=29942
Download the files appropriate for you depending on your running a x86 or x64 version of Windows