A guide to the most common computer security threats facing home and small business users in 2009:


A generic name for all malicious software. Some people include greyware which is software that’s annoying rather than dangerous.


A program designed to automatically copy itself from one computer to another so that it spreads. Viruses attach to other pieces of software or hidden inside images, games and music files. They usually travel from machine to machine by email, instant messaging or file transfers.

Although some viruses are harmless, most are disruptive or dangerous. The worst can stop a PC from working.


Another kind of self-replicating program. Unlike viruses, worms can automatically travel from machine to machine without being attached to other pieces of software. This means in addition to any other damage, they slow networks because they can consume bandwidth.


The name given to a program which looks harmless, but has an unexpected malicious purpose. Some start their mischief immediately, others may lurk for a time, possibly collecting data without the computer owner’s knowledge.


A program designed to collect information about a computer and its user that the spyware author can use to make money. Typically spyware may watch your web browsing and target pop-up advertising at you or divert you to other web sites.


A program designed to change a computer’s operating system, usually to hide the behaviour of other malware.


Software that collects keyboard input – possibly to collect passwords or important account information. Keyloggers send this information back to criminals allowing them to impersonate users and rob their online banking accounts or do other mischief.


Programs used to control, update or trigger activity in previously infected systems. Often hundreds or thousands of related botnets are triggered simultaneously to create vast amounts of traffic and take sites offline in what is known as a denial of service attack.


A way of getting undetected access to a computer system.


A computer controlled by another user to do malicious tasks online.


Unwanted email, instant messages or other form of electronic communication. Spam clogs email inboxes and the sheer volume of spam (as much as 95 percent of all email traffic) slows networks.


When someone fraudulently tries to get hold of important information such as passwords and bank account details by pretending to be a trustworthy source. Phishers may send authentic-looking emails asking for the data or with links to a fake website.


Strictly speaking this isn’t a threat, but an annoyance. It refers to any software that bombards you with unwanted advertising.

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