As with all BETA products, a good working knowledge of how to work around issues is essential:

Microsoft – Security Essentials v4 BETA release

QUOTE: Microsoft has released Security Essentials 4.0, the latest version of its lightweight, straightforward antivirus package. The bulk of the changes this time are under the hood. MSE’s “Automatic Remediation” now does a better job of quarantining threats on its own, for instance, no user intervention required. And Microsoft claims scanning performance and malware detection rates have both improved on the previous build.