LoveLetter - 'heart-breaker'

LoveLetter began appearing as a Valentine's-style message in e-mail inboxes all over the world in May 2000. The subject line was ILOVEYOU and the message read "kindly check the attached LOVELETTER coming from me." Attached to the e-mail was a program which, when the recipient opened it, immediately forwarded the message on to everyone in their address book, gradually clogging the world's networks. LoveLetter was one of the largest "mass mailers", and also used social engineering - the tactic of exploiting human nature - to devastating effect, in this case by pulling on the recipient's heartstrings.

Sql Slammer - 'fastest ever'

Sql slammer took advantage of a vulnerability in SQL Server - a piece of Microsoft software that many PCs use to access information stored on websites. Infected machines would in turn reach out to other computers via the internet and infect them - at extraordinary speed. Within 30 minutes of the virus appearing on January 25, 2003, it had infected 75,000 systems. The "install base" of the virus is understood to have doubled in size every 8.5 seconds. The world's computer networks were suddenly flooded with enormous volumes of traffic, and many businesses were unable to stop their systems grinding to a halt.

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Storm worm - 'one of the most prolific'

One of the most dynamic e-mailed based worms, which has had hundreds of variants since it first appeared in December 2006 and is still around today. The first version enticed recipients with the subject line 'New year's celebrations'. More recent e-mails containing the virus have purported to be about "Middle east tensions" or an "Earthquake in Beijing". Attached is a program, a link to a website or clip of a film, all of which can infect recipient's machine. Once installed, the virus can control the machine, meaning it can be forced to participate in spam or other scams as part of a giant network of zombie computers known as a botnet.

Netsky and Bagel - 'hacker wars'

Two more mass mailer viruses which surfaced in 2004 and were famous not only for their ability to infect machines but because of the battle that played out between their authors - "Netsky" and "Bagel". Both wrote programs which were spread via e-mail and which, when opened, would allow the author to control the victim's machine remotely. Each was fiercely proud of his or her own virus, however, and enjoyed trying to disable his or her rival's. An epic 'hacker battle' ensued. "Hey netsky f*** off you b****" was just one of many text messages Bagel included in the code of one of his viruses. 'Hey bagel, feel our revenge', Netsky replied.

Nimda - 'blended threat'

A dynamic virus which first appeared in October, 2004, and has baffled security experts with its multi-faceted approach to infecting the world's PCs. Sometimes it spread by e-mail, and was so cunning recipients did not even have to double click on an attachment for it to infect their computer, sometimes it embedded itself in web pages and attacked machines which visited them. It was the first virus to combine range of malware techniques and bundle them together, and at its peak, affected millions of PCs. "Virtually every business" had instances of the virus on their systems at some point, according to one researcher.

(Source: McAfee, a US-based security software company)

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