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What are computer viruses and how do you avoid them?

What is a computer virus header

Viruses are one of many threats your computer faces. We explain how you’re attacked and how to stay safe

Although the word virus is still used as a catch-all term for any threat to your computer, the truth is that there is a variety of different threats out there, which are collectively known as malware. It’s a growing problem, with cyber-criminals coming up with devious new ways of infecting your computer. The main reason for this growth is simple: cold hard cash. After infecting your computer, criminals can make money from displaying adverts, stealing your personal data (ID theft) and even remote controlling your PC, renting it out for illegal activity, such as sending spam.

To help you stay safe from all of them we’ve teamed up with BullGuard to produce this definitive guide, which has definitions for all of the threats you’ll face and top tips for avoiding them. It almost goes without saying that one of the best things you can do is run up-to-date security software, such as BullGuard Internet Security.


Definition It makes sense to start with traditional viruses, as they’ve been around for the longest and, as we’ve noted, are still commonly referred to for all malware. A virus is a program that, when run, attaches itself to a legitimate piece of software, infecting it. Every time you run that software, the virus reproduces itself, attaching to more applications and spreading the infection.

How do you get infected? Viruses typically need some kind of manual interaction to run them. This can be through downloading an infected application; social engineering, where someone tricks you into running a file, such as via an email; Macros in Word and Excel documents; and through auto-run applications on CDs and USB drives. Some viruses take advantages of flaws in common software and operating systems.

What do they do? Viruses are mostly malicious and can have a range of effects, from logging and stealing your private data, to deliberately causing harm and damaging your PC.

How to stay safe As viruses typically need action to run, staying safe is largely about being vigilant: never download and install software from dubious sources and always go directly to the software manufacturer’s website; do not insert unknown CDs or USB drives into your computer; never open an email attachment, unless you can be sure that it’s legitimate; and run a regular virus scan on your computer.


Definition While traditional viruses need to piggy-back on legitimate software, the advent of the internet has seen the introduction of worms, which run automatically and spread themselves over the web. These are much more dangerous than traditional viruses, as you’re at risk the moment you go online.

How do you get infected? While the first worms used email to send themselves out to your contacts list, requiring the receiver to open the attached infected file, the modern varieties are much worse. These spread by taking advantage of flawed operating systems and applications. In effect, worms seek out vulnerable computers and infect them silently.

What do they do? Worms are similar to viruses and they’re malicious. They can steal your personal data, allow cyber criminals to remote-control your computer for the purposes of sending spam and other illegal activities, or just damage your computer.

How to stay safe As worms are spread automatically, it’s important to make sure that you keep your operating system and all software up-to-date with the latest patches. Security software will protect you in real-time from brand-new threats.


Definition Rootkits are one of the biggest threats to modern computers, as they’re designed to hide the existence of malware on a computer and let the hackers upload even more when they want.

How to do you get infected? Rootkits are often designed to replicate automatically over the internet, looking for computers running out-of-date operating systems and software, and exploiting flaws in them to install automatically. Other Rootkits are bundled up with legitimate-looking software, tricking you into installing them (the Trojan Horse method).

What do they do? Rootkits hide and mask their existence, so you can’t see that they’re running on your computer. Even worse, they allow hackers to install additional malware (spyware, viruses and worms) on your computer, hiding its existence as well. This makes it hard to remove the malware from your computer. As BullGuard says, “Detecting a rootkit on a Windows PC is not unlike shining a flashlight at objects in a darkened room, and then trying to identify each object by the shadow it casts on the wall.”

How do you stay safe To stop a rootkit from installing itself automatically, you should ensure that you’ve run the latest patches and updates on your computer and its software. Be very wary of free software that you’re downloading and always go directly to a manufacturer’s website: do not trust dubious third-party download sites. Of course, run up-to-date security software to keep yourself protected in real-time.


Definition Spyware, as the name suggests, is software that monitors what you’re doing secretly, and can even send this information back to the application’s creator.

How to you get infected? Typically, spyware isn’t spread automatically but has to be installed manually. It can either be part of an application that sounds useful, such as a web accelerator, or an optional installation when you install other free software.

What does it do? At best spyware will just monitor your internet usage and send statistics back to the software’s creator, which they can then sell on. Some spyware is a lot more malicious, such as keyloggers, that record every keystroke, recording all of your private details and passwords. Spyware can also slow your computer down, max-out your processor, and even redirect your web browser to sites that you had no intention of visiting.

How to stay safe As spyware has to be manually installed, you should be very careful about what you put on your computer. You should never install software from unknown sources, and only visit the official manufacturer’s website; be wary of any free software that’s not from a manufacturer that you recognise; when installing any bit of software, check for any option to de-select any additional software from being installed; and run up-to-date security software with anti-spyware protection.


Definition Adware is software that’s designed to pop-up adverts on your computer, making the creator money.

How do you get infected? Adware, as with spyware, typically has to be installed manually. It can either come as hidden with a supposedly useful bit of software, such as a web accelerator, or it can be an optional add-on with legitimate software.

What does it do? Adware’s sole purpose it to put adverts on your computer. This can be popping-up windows with ads in, or through an ad-injector, which puts adverts into websites that you’re looking at. As well as being extremely annoying, adware can cause your computer to be unstable and it can cause websites to look strange or not load properly.

How to stay safe As Adware typically has to be manually installed, you should be careful about where you get your software from. Never download software from dubious site, only visit the manufacturer’s page; don’t trust free software that you’ve never heard of before; when you’re installing, look at for a checkbox to disable optional software from being installed; and run up-to-date security software to block the latest threats.

Fake AV

Fake anti-virus

Definition Fake AV is a growing threat and one that’s largely designed to fool you into parting with your money, with an increase in social-engineering attacks largely propagating it.

How to you get infected? Fake AV is usually installed manually, either because you’ve been tricked into doing so or because it’s bundled in alongside a seemingly useful bit of software. Commonly, someone will phone up and pretend to be from Microsoft, telling you that your computer’s infected and then getting you to download software onto your computer to ‘fix’ the problem. Some Fake AV is just triggered by visiting a website, with a pop-up designed to look like real software warning you of security problems.

What does it do? Largely, Fake AV doesn’t do anything malicious, but it reports thousands of ‘errors’ on your computer, which you then have to pay to have ‘removed’. However, there’s a real danger that alongside the fake part of the software is real, malicious software  (viruses, worms and spyware) doing something much more nasty in the background.

How to stay safe Be vigilant when someone phones you about computer problems: Microsoft and other companies do not make these kinds of calls; if you get one, do not install any software that they tell you to. You should be wary about installing free software from manufacturers that you’ve never heard of, and you should only download software from the manufacturer. Ignore any pop-up windows that come from your browser: these can’t tell if you’re infected. The only safe protection is software from a reputable security company, such as BullGuard.

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