Viruses, worms and trojans

Avoid the nasties that want to harm your computer

If you use your computer to communicate with others – on a network, the web or email – you’re at risk of infection from computer viruses. There are now more than 81,000 different viruses out there according to the McAfee AVERT Virus Information Library, and the number is rising each day. It’s highly likely that you’ll encounter malicious code at some point.

Infection can be expensive. It is estimated that PC viruses cost businesses approximately $55 billion in damages last year.

A virus is a program designed to alter the way a computer operates without the knowledge or consent of the user. Although no virus can cause physical damage to your PC, a virus can render your computer useless.

Are You At Risk?

How do you know if you’ve been infected? Here are some of the common warning signs:

•	Your system slows down suddenly
•	An unexpected message appears
•	Files or data are lost, corrupted or unexpectedly changed
•	Some files or data are unexpectedly sent via email
•	You see unexpected activity on your computer
•	Programs function incorrectly or not at all

Email viruses are common. You’ll receive a message that contains an attachment; when you double click on the attachment, the virus code starts running. The code sends copies of itself to everyone in your Outlook or Outlook Express address book. It might then start corrupting files on your computer.

Viruses aren’t the only examples of malicious code that you will come across. The most common threat comes from worms – programs with the ability to copy themselves from one computer to another. Worms normally propagate over the internet, and can expand from a single copy incredibly quickly. The MyDoom worm generated between 50,000 and 60,000 new copies per hour. Most email ‘viruses’ are actually worms; they don’t damage files on your computer, though they may fill your disk and memory with spurious copies of themselves.

The third member of this ugly roll-call is the Trojan horse. Trojans appear to be harmless but are malicious and can do a lot of damage. Some common uses of Trojan horse programs include rounding (carving off small parts of payments from a large number of accounts or transactions), making illicit payments to their host and stealing security information.

Steps You Can Take

The malicious code has to get into your PC somehow. The likeliest source is a file you download from the internet or one that an unsuspecting friend sends you. Increasingly virus writers are using automated programs to trawl for connected computers to sneak into.

A firewall is the best protection against this kind of intrusion. It’s a program designed specifically to control communication to and from your computer. A firewall prevents unauthorised access from the Internet to your computer.

Windows XP has always had a built-in firewall option, but Service Pack 2 extends its capabilities. SP2 turns on the Windows Firewall by default for every network connection that you use, so you’re always protected. You can also choose which programs may receive incoming traffic so you’re safe but still productive. SP2 has some other critical features to counter intrusions. For web browsing, SP2 extends Internet Explorer with a pop-up blocker and the ability to disable browser add-ons. SP2 also stops malevolent sites hijacking Internet Explorer.

Potentially dangerous attachments are now quarantined in Outlook Express and Windows Messenger but if you’re sure a file is safe you can still open it.

Service Pack 2 will automatically check for the latest updates for your computer and install them. Not everyone recognises the importance and ease of using this facility, however. So Service Pack 2 requires you to make a choice during installation, strongly recommending that you turn on the automatic updating feature before you do anything else. You should leave it switched on: it is vital that you have the best possible protection as soon as it is available.