I've been using Avast for the past few years and can't say enough good things about it. Came over from McAffee, which did an absolutely terrible job on system performance.
I'm surprised that no one has mentioned F-secure. When micr*soft has problems that's who they go calling on. They only have a short free trial. To get the best you need to pay abit.
the best antivirus is Windows 7 OS. It has soo much security, u will never think of installing third-party antivirus.