Microsoft Security Essential (MSE) is one of the best free Anti-Virus and Anti-Spyware software available for windows computers. Recently a public beta of MSE version 4 has been released and available for download. If you are interested then you can download it now.
This Beta version of Microsoft Security Essentials comes with the following new features:
1. Enhanced protection through automatic malware remediation: It'll clean highly impacting malware infections automatically, with no required user interaction.
2. Enhanced performance: It includes many performance improvements to make sure your PC performance isn’t compromised.
3. Simplified UI: Simplified UI makes Microsoft Security Essentials Beta easier to use.
4. New and improved protection engine: The updated engine offers enhanced detection with cleanup capabilities and better performance.

If you are interested then you can download and install it on any windows computer running Windows XP SP 3 or later. You can Download MSE Beta from Microsoft Connect website but you will need a Windows Live account to download it.