Microsoft Security Essentials 4 is now available as a free download.
This edition has been under beta testing for a while now. Users already running the beta are getting the latest build through Windows Update. I got it earlier today.
Microsoft has not released any significant changelog so we assume nothing major has changed.
The user interface has been changed a little bit.
Users of this security app would confirm that it requires little intervention and stays in the background.
Probably the best free antivirus solution for a Windows machine.