Know Your Enemy

Users tend to call every malicious program a virus, but the dangerous programs encountered these days are rarely viruses in the technical sense. More commonly, they are worms or Trojan horses. The general name for malicious software is malware.

Malware made its first appearance in the 1980s, and the first malware programs were viruses. Computer viruses attach themselves to other programs; when an infected program is run, the virus is run as well, infecting other programs or a computer's boot sector. Today, most programs are distributed on read-only CDs, and virus checkers can protect your PC's boot sector.

A Trojan horse is a destructive program that disguises itself as something else. Unlike viruses and worms, Trojan horses don't replicate themselves; they just do bad things when you run them. In the 1980s, they were often distributed as appealing-sounding games, but today a classic Trojan is rarely seen. More commonly, you'll see blended threats distributed as e-mail attachments whose payloads are worms or viruses. You should never double-click on an executable e-mail attachment, even when you recognize the sender. Executable attachments have the file extensions .com, .exe, or .vbs.

A worm can copy itself to other machines without human interaction. It does this via network security holes. For example, a worm can be implemented as a script embedded in HTML e-mail. You don't have to double-click an attachment to get infected; you just have to view the message in an HTML-enabled e-mail client. Until recently, you couldn't turn off HTML e-mail in Outlook Express. But if you're running Version 6, Service Pack 1 or later, select Tools | Options, go to the Read tab, and check the box labeled Read all messages in plain text.