Microsoft Security Essentials

As a no-cost service for genuine Windows PCs, Microsoft Security Essentials offers you comprehensive, state-of-the-art protection against malicious threats such as viruses, Trojans, spyware, and other malware. Made for Windows Vista and 7, it delivers quality protection backed by industry standards and is updated regulary with the most recent virus databases. It is simple to use and understand, while also not getting in the way of the performance of your PC. It can be customised with settings, like real time protection, setting the limit of using processor resources and programs and files not to scan.
                                                            Download link:
                  http://www.microsoft.com/en-us/security_essentials/ProductInformation.aspx