A worm is a virus that can duplicate itself and attach to other programs on your computer.

A worm can copy itself across entire networks, and usually worms are created to perform some kind of malicious deed, use up resources and even shut down the system it attacks.

In extreme cases, worms can affect computer networks world-wide and drastically slow down the internet.

What worms do

Worms hide in your operating system, in parts that are not frequently accessed, and often go un-noticed until they start to slow down your computer or adversely affect other programs.

As well as replicating itself, a worm can be programmed to carry out other tasks such as attaching itself to your outgoing e-mail or deleting your files.

Worms have the ability to send themselves to everyone in your e-mail address book - and then to everyone in their e-mail address books. So they can grow and spread rapidly.

Internet worms spread from computer to computer

One common action performed by worms is to create a 'backdoor' way into the computer it infects. Once they have done this, people who send spam can do it from the infected computer.

People who create worms have been known to sell lists of infected computers and even to blackmail companies with threats of attacks.

Famous worms

Some of the most damaging worms struck in 2003-4, and include 'Mydoom', 'Sobig' and 'Sasser'.

The recent 'Sasser' worm affected computers running on Windows 2000 or XP, and caused machines to continuously crash and restart. It drastically slowed the internet as it sent itself all over the world from computer to computer.

The mobile phone virus 'Caribe' was a worm, as it sent itself from one mobile phone to another across a network.

Unlike other types of virus, a worm doesn't require you to do anything (such as opening an attachment) to get to work.

How to protect your computer

So what can you do to protect your computer?

Prevention is better - and easier - than cure, and following these simple (and hopefully familiar) guidelines can help protect you:

• Make sure you have a firewall installed. There is one built in to Windows XP.
• Never open an e-mail attachment unless you know the person sending it to you, and you are expecting an attachment. If you aren't sure, check with the sender or delete it.
• If you download programs from the internet, make sure they are from sources you trust
• Use up to date anti-virus software
• Keep your operating system up to date

One final word, you may well receive e-mails forwarded to you by friends containing 'virus warnings'. These are nearly always chain letters and hoaxes.

Before you agree to 'forward this letter on to all your friends and family', a quick visit to an internet security website such as Symantec or Mcafee, or a web search engine will easily identify hoax warnings.