Viruses

A virus in biology is an organsim that enters the body, latches onto cells and replicates itself over and over again, usually causing nasty side effects, eg: the common cold.
A computer virus is similar, except it attaches itself to computer programmes rather than people. Origionally they were fairly harmless, one caused letters on the screen to fall to the bottom; annoying but easy to fix. Later viruses became considerably more damaging, causing huge time, financial and patience loss.

Spread
Most viruses are spread via email attachments, some have even been designed to completly take over someones email system to replicate itself and send itself to other people.
Other ways include removable devices like floppy discs, CDs, DVDs or mempry sticks. Discs from large companies are unlikely to contain viruses, but home-burnt ones may.
Usually there is no immediate effect and people do not notice the virus until it becomes a problem.

Removal
The best way to remove a virus is with an anti-virus software, probably the most famous of which is Norton. These work by looking for patterns that match with known viruses and alert the user, giving a choice of ignoring, deleting or quaratining. As new viruses are being created all the time, they will need regular updates, either over the internet or by buying the latest version whenever it comes on the market. They usually cost, but it is worth it.
Anti-virus software can have disadvantages sometimes though, installing some programmes can be a pain, as the software will mistake the installation as a virus spreading through the system and prevent it. This is why most softwares tell you to deactivate your anti-virus software during installation.
Another way to delete the virus is manually, by directly going into the affected files and deleting them. This can cause problems though, especially if the file in question is an important one, and it only works if the virus is only installed there and hasn't spread yet.

Prevention
Viruses can be prevented from taking control by following a few simple steps:

• using anti-virus software to scan your system daily or weekly
• update the scanning software regularly
• not allowing people to load applications \ executable files on your system
• using special filtering software to prevent downloads of computer programs
• scanning all incoming e-mails
• not opening suspicious emails or attachments
• avoiding software from unreliable sources
• backing up data regularly so you can recover your data with clean copies

Other computer pests

Trojans: Applications that are disguised as useful programmes, eg: games, but really are doing something nasty. They can: log key strokes and send them,to capture passwords etc; allow someone to take control of your computer when you're online; take screenshots and send them, to capture personal information like bank details; and many other malicious tasks.

Worms: Lika a computer virus but will spread itself along a network. The main problem with them is the huge amount of bandwidth they use up, slowing networks right down.

Adware/spyware: These latch onto a computer and take notes on which ads the user clicks on, and then send more to the computer. They aren't really damaging but can be a right nuisance and tricky to remove.

Viruses and the law

The law states under the Computer Misuse Act of 1990 that a virus is an 'unautherised modification of the contents of a computer, impairing the operation of any programme or reliability of data'. Anyone caught distributing viruses knowingly faces an unlimited fine and up to a five year prison sentence.