Definition: "antivirus" is protective software designed to defend your computer against malicious software. Malicious software, or "malware" includes: viruses, Trojans, keyloggers, hijackers, dialers, and other code that vandalizes or steals your computer contents. In order to be an effective defense, your antivirus software needs to run in the background at all times, and should be kept updated so it recognizes new versions of malicious software.