I use NOD32 Antivirus (not their Smart Security), have done for 6 years (or more). I like it because compared to other paid antivirus it sits there quietly, preferring not to bug you endlessly and doesn't stick it's branding everywhere it can (Looking at you McAfee).
Instead it just gets on with the job it's designed to do without trying to get too clever or overwhelm with extra needless functionality. When it needs to tell you about an infection it does so in an easy to understand way, showing a native-looking Windows dialog with easy to pick choices, not some confangled counter-intuitive brand heavy UI like others do.
It seems lately that Antivirus applications are competing to see who can bolt the most shit on to your OS, including browser toolbars and extras like anti-spyware and anti-spam filters and anti-this-that-and-other. Many of them even alter (and sometimes break) the functionality of other applications without telling you, which is fucking annoying to unpick (still looking at you McAfee). NOD32 does none of this, it runs in the system-tray and mines it's own business.
EDITED: 12 Jan 2014 14:16 by MATT