I first installed Microsoft Security Essentials last year on office computers because I wanted some minimal antivirus software without the bloated mess that most commercial antivirus systems produce. It seemed to work well enough so I installed it at home (for several decades I've been running with no antivirus at all—it's not dangerous if you know what you're doing). On rare occasions it is a bit intrusive but for the most part it is quiet and reasonably effective. I refuse to install any bloatware antivirus on any machine of mine.