Microsoft Security Essentials

Microsoft Security Essentials is a favorite of mine with great detection rates, particularly for rootkits. Even more impressive is the fact that Microsoft Security Essentials has very few false positives, is light on resources and is good at removal of existing malware. MSE is a great choice for average users because of the minimal user interaction required. It automatically updates and removes threats. No registration is needed, apart from a quick validating of Windows, and there are no nag screens or advertisements. The main downsides are the slow scan speeds and the lengthy amount of time it takes to quarantine malware, though in my opinion these are rather trivial concerns, as for example a scan just be run overnight. MSE also is not available in certain countries so users there will have to look elsewhere. Note that Microsoft Security Essentials requires a genuine copy of Windows to install.