Microsoft Security Essentials 4.0.1526.0:

- New and improved protection engine.
- Enhanced performance.
- Simplified UI.
- Enhanced protection through automatic malware remediation.
- New Microsoft Active Protections Services (MAPS).

Microsoft Security Essentials 2.1.1116.0:

- Fixed Bugs.

Microsoft Security Essentials 2.0.657.0:

- Network Inspection System
- New antivirus engine.
- Windows firewall integration.
- Microsoft AntiMalware Script Scanner.