Ask most home computer users what they consider to be the biggest security threat to their PC and they're likely to say viruses. It's true that viruses pose a very real threat but, with decent antivirus software installed on your computer, you can minimise the risks.

If you use a dial-up connection to surf the web, though, you may find that you are at risk from another, much more financially damaging menace: diallers, and even broadband users need to take some precautions.

In this feature, we'll take a look at the threat posed by diallers and what you can do to minimise the risk to your PC. Diallers often install themselves on your PC without you even realising it and work by changing the phone number that you use to connect to the internet.

This might not sound like a huge problem, but diallers usually connect your PC to a premium-rate number and you could be in for a nasty shock when the next phone bill arrives, as many calls are charged at a heart-stopping £1.50 per minute.

Luckily, though, there is plenty of software available that will help you locate diallers on your PC and get rid of them. As some of the most popular of these packages are available free of charge, we decided to test whether paid-for software actually offers any better protection or if free software will offer a good enough defence.

Dial-up phoneys
Before we start looking at the various anti-dialler packages currently available, let's start by taking a closer look at what diallers are, how they install themselves on a computer, and what the end result is likely to be.

A dialler is a small program that resides on your PC, often invisibly. Once the software changes the number your PC dials to access the internet, you may well find yourself paying premium prices instead of your usual local or free-phone number whenever you go online.

Not all diallers are malicious. Indeed, many are a legitimate pay-as-you-go method of accessing website content. These diallers will warn you before they're installed, tell you exactly how much extra you'll be paying to access the internet using them and have a clear uninstall process. It's the less scrupulous diallers that are more of a concern.

Although these diallers still need your consent before they install themselves on your computer, the people who create them use underhanded tactics to trick users into agreeing.

For example, if you visit a site that immediately bombards you with hundreds of pop-up windows, a dialogue box might also appear asking you if you want to speed up your PC. Even if you click on the 'No' button, it will most probably be a fake 'No' button and the dialler will install anyway.

Anyone using a broadband connection will be pleased to hear that diallers don't pose much of a threat to them. Since an always-on connection doesn't dial a normal phone number whenever you go online, premium-rate diallers have no effect.

However, if you have upgraded to broadband, it's sensible to disconnect any internal or external modems by unplugging the cable between the PC and the phone socket that is used to carry your dial-up connection.

Tracking the enemy
Now you know the risks that diallers can present, you'll want to know how to find out if one's been installed on your PC. The bad news is that unless you've got specific software installed on your PC (which we'll come to later), there's a good chance you won't immediately realise a dialler has been installed.

Your internet connection will remain active and you'll be able to surf the net just as before. The change will occur when you disconnect and redial. If you are faced with an unfamiliar home page when you connect to the internet(more often than not containing adult material) you may well have a dialler.

You can scan your PC for diallers using spyware detection software but the best way to prevent a dialler being installed is to be cautious about the websites you visit in the first place. If you log on to a site that instantly flashes up a host of pop-up windows and adverts, be wary.

You may want to install software that will stop these pop-up windows appearing altogether. A good example is Popup Blocker which is free to download.

It's also good practice never to click on any buttons in pop-up windows, even if they're labelled 'Close' or 'Exit'; sometimes, the familiar 'X' button in the top-right corner of the window will be faked and instead of closing the pop-up it will actually install software on your PC.

The best way to close pop-up windows is not to click on them at all, but simply to hold down the Ctrl key and press W to close the window. You can also ask your phone company to block access to all premium-rate numbers, although some charge a fee for this.

The good news is that there is plenty of security software available on the internet to help you remove diallers and we'll cover the features offered by them in a moment. By scanning your PC regularly using one of these tools, you'll soon find out if a dialler has been installed on your PC. In general, it's best to run a scan at least once a week, but if you've just visited a site that set off alarm bells it's best to run a scan straight away.

Freedom fighters
To test the different packages on offer, we set up a PC with a fresh installation of Windows XP. We then installed a selection of diallers and ran each piece of anti-spyware software to determine what sort of protection each package offered. We also explored the various features of each one, paying special attention to the ease of use of each package.

Ad-aware SE Personal and Spybot Search and Destroy are two packages that are free to use and will detect a variety of the latest diallers.

Ad-aware SE Personal, from Lavasoft, has been around for a long time and is still one of the most popular packages currently available. Even if you're a complete beginner it's easy to use. You can start a scan of your entire PC with just two clicks of the mouse, and removing any diallers or spyware is an easy task.

If you want to perform more advanced scanning, Ad-aware SE Personal has a wide range of manual settings. For example, you can specify which drives and folders you want scanned and exactly how detailed you want the software's report to be.

If there are certain items that you know you need but are constantly being picked up during scanning, you can instruct Ad-aware SE Personal to ignore them in future. To keep the database current, the Webupdate button will connect directly to the Lavasoft website and download the latest list of diallers and spyware. Alternatively, you can request Ad-aware SE Personal to check for updates automatically on start up, which we would recommend.

Spybot might not be as aesthetically pleasing as Ad-aware but it's just as effective. Thanks to the installation wizard, setting the program up is no problem at all but we would recommend spending 10 minutes going through the supplied tutorial.

Once you've performed a scan, every item that Spybot identifies can be explored in more detail. For example, you can view the name of the company that produced the detected item and the recommended action that should be taken.

In an attempt to reduce the amount of spam email you receive, Spybot includes an online feature that automatically sends opt-out emails to the marketing departments of some of the biggest advertising vendors. It won't completely eliminate spam on your PC, but it should at least reduce the amount you receive.

Spybot also features a tool called Immuniser, which automatically blocks the most common spyware applications from being installed.

Active scanning
If you're willing to shell out a bit of cash, two popular commercial packages are Ad-aware SE Plus and Webroot Spy Sweeper. Both of these offer similar features to the free versions mentioned above but also include a real-time scanning system.

This means that your internet connection is constantly being monitored. If a dialler attempts to install itself it will be stopped in its tracks and you'll be notified.

Ad-aware SE Plus costs £17 per year and has exactly the same interface as the Personal edition, but it also gives you access to the real-time Ad-watch monitoring tool. The level of protection Ad-watch provides can be tailored to suit and it's also able to detect possible attempts to hijack your browser and other suspicious processes.

Spy Sweeper is a well-designed program that houses very similar features to the other packages mentioned here. The Active Shield component will constantly monitor for diallers and other dangerous applications when you're browsing, while deep scans of your entire PC can be scheduled to run at specific times. You can try out Spy Sweeper free of charge but if you want to receive essential updates you'll have to subscribe to the service.

As you'd expect, all of these packages are capable of detecting far more than just diallers. They will also pick up a wide range of adware, spyware and other malicious applications that can slow down your internet connection and invade your privacy.

Best defence
During testing, we found very little difference in the number of diallers and spyware picked up by each of the packages. Since the dialler and spyware databases for each product are updated regularly, they all offer a similar level of protection. Each package also offered an instant web update service from within the program itself, making updating both quick and easy.

While the two free packages, Ad-aware SE Personal and Spybot, offer adequate protection if you scan your computer regularly, they don't feature any real-time monitoring.

Both Ad-aware SE Plus and Spy Sweeper include tools that will constantly scan for diallers and other suspect programs while you're surfing the internet. This has the advantage of stopping dangerous applications before they're even downloaded.

If you're careful when visiting unknown sites and regularly scan your computer, we believe the free software we've looked at here offers an acceptable level of protection. However, if you've been a victim of a dialler before or just want to make sure you've got the highest level of protection possible, both Ad-aware SE Plus and Spy Sweeper are reasonably priced, with the latter offering a few more advanced features.

A fighting chance
Should the worst happen and you find out that a dialler has caused your phone bill to rocket, you should first contact your phone operator. Explain the problem and, if you don't get anywhere, your next port of call should be ICSTIS, which is the independent body responsible for regulating the UK premium-rate telecoms industry. The phone number is 0800 500 212.

ICSTIS can fine or cut off companies that flout its regulations but cannot compensate you or force offending companies to pay up. It can only give you the address of the service provider so you can claim compensation from them. Some pay up and some don't, and the bad news is that, under existing laws, you have to pay the phone bill regardless.