There have been many high-profile news stories about computer viruses in the past few years and all responsible computer users will be aware that they need some sort of antivirus software on their PCs to ward off the worst of these threats.

Unfortunately, though, a new security threat has arisen over the past couple of years and grown so rapidly that it's now just as much of a concern as viruses to PC users.

We're talking about spyware, sometimes also known as malware, which is short for 'malicious software'. This is software that can install itself on your computer, or even trick you into installing it yourself by offering free goodies such as music or video downloads.

Once installed, however, this software can turn into a serious threat. At its mildest it may swamp your computer screen with a barrage of unwanted adverts. At its worst it has the potential to steal important personal information such as the details of your credit card or online bank account. In this feature we will show you the measures you can take to prevent these irritating spies from disrupting your PC.

Know your enemy
Spyware is a general term that is often used to refer to a number of different types of nuisance software. The first one we'll look at here is known as adware. This is software that installs itself on a computer and then bombards the user indiscriminately with adverts.

Most adware attaches itself to a web browser such as Microsoft's Internet Explorer, so adverts tend to appear while you're using the browser to access the internet.

However, some adware software runs in the background all the time an infected PC is switched on and may bombard you with ads even when the PC isn't connected to the web. Sometimes so many ads will pop up on the screen that they can prevent you from doing much else with the computer besides closing these ads down, which is obviously a real problem.

There's also a particular type of adware that 'hijacks' a web browser's home page, so that the browser displays a particular web page every time you launch the browser software, usually the web page of whichever company developed the adware. This will then bombard you with more ads.

Sites that offer free software or pornography are common sources of adware, which may be hidden in the program that first caught your attention. If you notice your web browser's home page or toolbar has been changed in some way shortly after downloading a piece of software or if browsing the web becomes slower, it could well be a telltale sign that adware has been installed on the PC in this way.

Note that as malware is often downloaded as part of another program, it won't necessarily show up in the Add or Remove Programs list that you can access through the Control Panel in the Start menu to check what software is installed on your PC.

Also beware of file-sharing services on the internet, some of which are notorious sources of adware of this kind. Typically a service will state that downloading its special file-sharing software will give you access to a vast library of music and video clips. But while this software may indeed allow you to share files with other internet users all over the world, it may also start to fill your screen with annoying pop-up ads.

Fortunately, it's not too difficult to deal with adware. We'd recommend downloading two shareware programs called Ad-Aware and SpyBot.

These two programs can sniff out most types of adware and spyware and remove them from a PC but they won't actually stop adware from installing itself on a computer. It's worth downloading and using both programs to make sure that you've cleaned out the PC as thoroughly as possible.

Remember to run these programs weekly to make sure they catch any new adware or spyware that may sneak onto your computer. You must also update the software before running it so that the application can scan for the newest known threats.

It's also worth pointing out that most internet service providers (ISPs) are more than happy to offer their customers advice on how to deal with spyware. Take a look at your ISP's website to see what help is available.

Some of the bigger ISPs provide their own anti-spyware software free of charge - anti-spyware tools are included in the AOL Safety and Security Centre software package, for example.

I spy
It is important to make the distinction between adware and actual spyware, which we'll come on to now. Like adware, it's likely that you'll download spyware unintentionally, perhaps when using file-sharing services or downloading free software that often actually claims to protect a computer from viruses or block pop-up ads.

As the name implies, though, spyware will actually spy on your computer usage to gather information that can be sent to whoever created the software. Some spyware programs are more of a nuisance than a real threat.

They may spy on you as you browse the web to see what sort of websites you're interested in, and use this information to target you with a few ads.

Accusations have been levelled at Google's Gmail webmail service about the way it monitors users' emails to target them with advertising. When you send emails via Gmail, you'll sometimes find small ads placed to one side of the main message.

What has happened here is that the Gmail computers have read your messages and inserted what they consider to be 'relevant' advertising. A user sending digital photos to friends may, for example, find they are receiving ads for online photo-printing services.

But, to be fair to Google, it is providing the Gmail service for free, and it does tell people about its use of targeted advertising. It's up to you to decide whether you're prepared to tolerate this form of advertising in return for a useful and free email service. Provided you are made aware of this before you download software, you can make your own decisions about what you are prepared to put up with.

Some spyware programs, though, are more sinister in their activity. There's a particular type of spyware known as a 'keylogger' that can record every single key that is pressed on a computer keyboard and the order in which they are pressed.

If you use an online banking service these keylogger programs could be used to record your account number and password to gain access to your bank account.

That may sound a bit far-fetched but these programs are out there and, while they are thankfully still rare enough to pose a very remote risk to the majority of PC users, the threat should nonetheless be taken seriously.