Analysis Today, 19 January is the 20th anniversary for the appearance of the first PC virus. Brain, a boot sector virus, was let loose in January 1986. Brain spread via infected floppy disks and was a relatively innocuous nuisance in contrast with modern Trojan, rootkits and other malware. The appearance of the first Windows malware nonetheless set in train a chain of events that led up to today's computer virus landscape.

Boot sector viruses ceased to appear when floppy discs went out of fashion but they continued to be a nuisance between 1986 to 1995, when internet technology started to penetrate the consumer market. These types of viruses relied on people to exchange infected discs and virus outbreaks often took months to spread.

The creation of macro viruses, which exploited security weaknesses in Microsoft word and other applications, meant that malware outbreaks peaked after days instead of weeks and months. Macro viruses ruled the roost for around four years between 1995 and 1999 before email became the main vector for viral distribution.

Harnessing the internet meant that the time it took the first email worms, such as the Love Bug, to spread dropped from days to hours. Email worms such as the Love Bug and Melissa caused widespread disruption and confusion in 1999 before they were brought to heel.

By 2001, network worms such as Blaster were created that automatically and indiscriminately infected Windows PCs without adequate protection. Email and network worms remain a problem today but the greatest problem these days is posed by key-logging Trojans designed to snoop on user's private information, such as online account details, and the many strains of malware that turn infected PCs into zombie drones under the control of hackers.

The biggest change over the last 20 years has been in the motives of virus writers rather than in the types of malware they've cooked up, according to anti-virus firm F-Secure.

"The most significant change has been the evolution of virus writing hobbyists into criminally operated gangs bent on financial gain," said F-Secure's chief research officer Mikko Hypponen. “This trend is showing no signs of stopping."

"There are already indications that malware authors will target laptop WLANs as the next vector for automatically spreading worms," he added. ®