Microsoft Security Essentials is a free anti-virus program.
All of the free anti-virus programs out there have usually pretty quickly fallen out of favour with me, in particular the previous community darling AVGFree.
Having a free option from Microsoft, that performs extremely well with minimal resource usage, is a boon.
For privacy reasons, you might want to opt out of the Microsoft SpyNet. Opting out is a very manual process though, with instructions here
