Old news by now (has it been sitting since Nov 2011).
Been running the x64 beta version on my home system with no ill effects. YMMV.
More info below.
Microsoft Security Essentials 4.0 Beta Available to Download - Windows7hacker
Free Download Microsoft Security Essentials 4.0.1111.0 Beta - Free Antivirus for Windows - I found this location to download the installation files from rather than register via the Microsoft links previously provided. I did grab the files both from my Microsoft registration and these and checked them both (HashMyFiles: Calculate MD5/SHA1/CRC32 hash of files). All hashes at the time matched.
Windows Defender Offline Beta - Bink.nu
AppRemover - OPSWAT - “Uninstall & Remove McAfee, Symantec, Norton, AVG, Avast & More Antivirus and Security Applications and Programs”