According to the Microsoft Download Center:

If you’ve already have it installed, you should be automatically updated. If not, head to the Microsoft Download Center or the Microsoft Security Essentials site. It will run on XP and above. There is a 64 & 32-bit version available. The new version looks cosmetically the same, but includes extra protection and stability. Editor Note: Keep in mind Windows Security Essentials doesn’t work on Windows 8. Microsoft has included MSE and Windows Defender. I’ve been running version 4 for about a week now, with the same impressive results. Coupled with their msert stablemate, SuperAntisSpyware Free and MalwareBytes Free I’ve run into no problems so far. To dodge shonky websites I use WOT,but I’m still not totally sold on that service. To add further protection I sometimes check downloads with Virus Total or Jotti. ClamWin also does a reasonable job. All good things in moderation, I say! Starting with Windows 8, Security Essentials will be built right into the OS and it won’t be a separate install any longer. MSE – freeware done right. Comment Name * Email *

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Microsoft Security Essentials 4 0 Released - 40Microsoft Security Essentials 4 0 Released - 9