What do the experts say about it?

Dr Fred Cohen, early virus researcher - "Viruses: a program that can infect other programs by modifying them to include a... version of itself."

How does it get there in the first place?

Viruses are all over the Internet, attached to files you can download. Sophos (an anti-virus software producer) estimated there's over 100,000 out there. Generally, the author of the file won't even realise they're distributing the virus, but if you get infected, you'll soon know about it. Anything that you can do with your files, so can the virus - changing, deleting, renaming, e-mailing...

Trojans are viruses that generally can't replicate themselves but are just as destructive, often opening up your computer for hackers to control. You normally pick them up from malicious files downloaded from websites. Porn dialers are trojans that hijack your dial-up connection, changing it to a premium rate number without your knowledge, which could mean a phone bill for thousands of pounds at the end of the month.

But how does this affect me?

These are your personal files we're talking about. While it might not matter if you lose a picture of your dog, viruses could delete important documents like pieces of coursework, or even cause your operating system to stop running. And Trojans, well... do you want a hacker to be able to remotely control your PC from the comfort of his armchair? Remember, anything you can do, they can do too - that includes viewing your webcam.

Get it Sorted

Trojans and viruses aren't just annoying, they're serious destroyers of personal data. Follow the advice below to reduce the threat to you.

• Install anti-virus software
• Install anti-spyware software
• Make sure your firewall is enabled at all times
• Don't open attachments from people you don't know
• Follow the Sorted top 10 rules to privacy online

Find out more

For further reading on trojans and viruses, take a look at the following sites:

• Information on the ILOVEYOU worm
• More information on computer viruses at Wikipedia
• Guardian Unlimited - Internet rogue dialer report