Microsoft have added Security Essentials to the free Windows Live suite and I’ve recently installed this. Security Essentials provides virus and spyware protection in an unobtrusive way.
I previously had been using Norton anti-virus but had many unexplained issues e.g. during system idle time my machine (64 bit Windows 7 i7) would crash and often during start up I would get the guest profile (rather than my user profile).
Since I replaced Norton anti-virus with Microsoft Security Essentials these issues have not appeared.