A friend called me the other day. She's an independent bookkeeper who works for many small businesses, usually in their offices on their computers. She's often their first line of tech support, even though that falls way outside her job description.

One of her clients' computers had been acting funny. She loaded up some anti-virus software, and sure enough it told her the machine had a Trojan. She cleaned it, restarted, and there was the Trojan again. A few attempts later, the Trojan was still there. That's when she called me.I assured her she'd done exactly what she should have and advised her to call in her client's professional tech support person. She gave a dissatisfied sigh and asked, "Why would anyone DO this?"--why would anyone write and distribute malicious software? I didn't have a good answer.

What I do know is that malware has gotten much more dangerous in the past 20 years. Early PC viruses weren't particularly harmful, and they spread via floppy disks--that is, slowly. Then they started to get smarter, attacking different types of programs, self-mutating to avoid detection, and encrypting themselves to make removal difficult. But still, they were outside the knowledge of the general public.

That changed with the much-hyped Michelangelo virus, set to strike on the anniversary of the painter's birthday in 1992. I didn't get the virus--at that time I was still safely working on a CompuGraphic typesetting machine rather than a PC. (Hard to believe, isn't it?)

The first PC virus I caught was 1995's Concept Word macro virus, something I would encounter again and again over the years. It didn't seem to do much and was easy to clean, but it was scary seeing that ominous dialog box pop up telling you your file was infected. Even then I remember thinking, "Why would anyone DO this?"

Mass-mailers like Melissa started spreading. We were told, "Don't open suspicious e-mail attachments." Then worms that were triggered simply by opening an e-mail appeared. We were told, "Don't open e-mail messages from anyone you don't know." (That wasn't much of a defense: Since many of these worms used Outlook address books to spread, the messages came from people you knew.)

The list goes on, with each generation of malware outdoing the last. Like a bad accident, it's abhorrent yet fascinating to watch how viruses and worms have evolved over the years. That's why we've put together "20 Years Of PC Viruses," an in-depth package of articles that puts malware under the microscope. You'll find a history of PC viruses (don't miss the cool Flash timeline), a first-person reminiscence of the early days of virus-fighting, and advice on protecting your business from malware attacks. The most popular story with our readers is "The 10 Most Destructive PC Viruses Of All Time." My favorite segment is the image gallery showing screenshots of some of the more interesting-looking (though often not harmful) viruses over the years.

What was your first encounter with viruses, worms, or other malware? Do you remember Brain, Tequila, or other early viruses, or was your introduction to malware more recent? Share your stories in the comments area below.