The free security program from Microsoft called Security Essentials protects our PCs from virus, trojans and other malwares. Microsoft has been updating the software constantly and now its on v4. Soon you will be getting notification in Windows Update program where you can update the MSE.
Here is the response from Microsoft on this v4 update,
This update to Microsoft Security Essentials is part of the standard product update release cycle. This service update contains product enhancements to improve performance and user experience. We have improved on Microsoft Security Essentials’ Automatic Remediation, which will automatically quarantine highly harmful threats without prompting the user to take action as well as minor bug fixes in set up, malware remediation and other areas.
If you are not using MSE, give it a try here.