Viruses: Viruses are sections of program code that attach themselves to other programs; they are often destructive and even when they aren’t they slow your computer down. In general, a virus is harmless until someone actually runs the program it is attached to. Viruses are generally spread when people share files with each other, often in email attachments.

Worms: A worm is a kind of virus that spreads itself. Once activated, a worm will use your computer’s normal functions to spread copies of itself. A very common method is for a worm to send a copy of itself to everyone in your email address book. Their computers will then become infected and so the infection will spread. Worms which replicate exponentially like this can cause severe disruption to email systems because of the weight of traffic they generate. Some worms have been known to open security breaches on infected systems, allowing hackers to gain access. (See below).

One of the problems associated with worms is spoofing. Many worms can not only send copies of themselves to addresses gathered from the address book on the infected machine, they can also make it appear that the messages came from one (or more) of those addresses, rather than from the infected machine itself.

Trojans: These are destructive or damaging programs that appear to be benign. They often arrive as innocent email attachments (which might appear to be a photo, web link or text file, for example). They usually appear to be from an apparently innocent source, but when opened they can cause all sorts of trouble. Some are simply a nuisance, changing you desktop settings, for example; some are known to delete or change files; and some are known to open your computer to attacks by hackers, or even to collect and send to hackers sensitive data, such as passwords.

Hackers: These are people (or automated systems) that attempt to gain access to your computer, usually to gather sensitive data such as passwords, credit card numbers etc. which can then be used for nefarious purposes.

Spyware: Spyware is a kind of program that collects information about how you use your computer, and sends it to a hacker. Spyware is usually installed on your computer without you knowing, often as part of the installation of another program that you have chosen to use (Usually programs that you get for free). Most spyware is used to collect commercial information, such as your browsing habits, to enable targeted advertising, but it can be far more malicious. At best, it slows down your computer and internet connection.

Adware: Like spyware, adware collects information about your browsing habits so that the adverts that appear in your web browser can be targeted to your interests. The difference between adware and spyware is that the perpetrators are usually up front about it. Many programs that you can download from the internet come with the option to try it out for free and the price you pay is to allow advertising. When you pay for the program you are given a key (ie a code number) to enter into the program and the adware component (and the advertising) is removed. Removing the adware without paying for a key will often stop the program from working at all.

Occasionally the licence agreement for a program will tell you that it includes spyware – however, most people do not read licences and anyway, they are often written in gobbledegook!

Diallers: These are malicious little programs that hijack your internet connection and redial it (silently) to a premium rate number. They tend to be linked to porn sites, which may often pop up unasked. If you have a broadband connection you are unlikely to have problems with diallers, since broadband connections do not use telephone dialling.

These sorts of software, which install themselves on your computer when you would rather they didn’t, often without telling you, are collectively known as malware.