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Best Free Antivirus: Windows Defender and 5 free alternatives

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Antivirus

Antivirus doesn't have to cost the Earth: we have five free alternatives to paid-for antivirus in our brand-new guide.

Even if you consider yourself to be a responsible web user, you should be using security software. Thanks to the proliferation of contaminated advertising iframes, opportunistic malware-laden spam and drive-by downloads that can affect even the most legitimate, upstanding and popular of websites, nobody is completely safe.

However, expensive anti-virus suites aren't your only option if you want to stay protected. While these paid-for packages often include a wealth of useful extra features, the core protection – real-time and on-demand virus blocking and scanning – is offered by their free counterparts too.

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Windows 10 comes with Windows Defender anti-virus already installed, and we've included it in our group test as a point of comparison for all the other software on review. While paid-for anti-virus software is beyond the scope of this group test, we hope to provide you with the information you'll need to make an informed assessment on whether or not a pre-installed commercial package, such as those often bundled with laptops, will be more or less effective than its free competitors.

Detection engines

The key part of any anti-virus software is its detection engine, which uses a combination of a database of known threats and analysis of an unknown program’s behaviour, known as “heuristic analysis”.

Detection engines are at the core of most anti-virus companies' business, and you'll generally find that a company's free products use the same engines as their paid-for products, although the latter may be equipped with additional features, such as firewalls and system optimisation tools that boosts performance further.

Some companies license their engines to others. For example, in this group test, Qihoo 360 Total Security can optionally use both Bitdefender and Avira's scanning engines in addition to its own. Multiple scanning engines, whether licensed or produced in-house, typically increase the likelihood that an anti-virus product will successfully pick up anything nasty trying to attack your system. However, it’s also very likely to slow down your PC due to the sheer number of engines running at once.

Similarly, while most software defaults to an optimised scan mode that checks the files most likely to have been compromised, running a more thorough scan will take longer but might find infected files that would have otherwise gone unnoticed. Running a deep scan on a semi-regular basis would be sensible.

Testing & performance

In this group test, we've relied on results produced by the respected AV-TEST security lab to assess the performance of each anti-virus program in correctly identifying malicious files and websites and avoid false positive detections of benign software as malicious. AV-TEST also assessed the impact of anti-virus software on system performance.

We've used test data on a per-engine basis, rather than an identical product basis. So, for example, we've used results from AVG Internet Security 2016 to help draw conclusions about the AVG Free 2016 – which uses the same engine – and used other test resources to confirm comparative performance. We’ve therefore had to make assumptions that the free and paid-for versions use identical engines, but given most paid-for software simply adds extra features instead of a whole new engine, this is a reasonably safe assumption to make.

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The test data we've published was collected by AV-TEST during March and April 2016, using the most up-to-date versions of the anti-virus software available. AV-TEST carries out both real-world testing, in which systems are exposed to live contaminated websites and emails, and reference-set tests, in which several thousand malicious files collected in the previous four weeks are introduced to the system.

This time around, the software was exposed to 164 live malware attacks and 22,795 reference samples. AV-TEST publishes percentage scores showing how effectively each anti-virus suite performed in these tests, which are then used to produce a score out of six based on how successful the software was at defending a system compared to the industry average.

We'll directly quote the percentage of malware successfully defended against. It's worth noting that there can be fairly dramatic performance differences from month to month. This depends on a given anti-virus package's ability to detect the malware that's prevalent at any given point, and the speed with which its signature database and heuristic detection rules are updated to reflect current malware. Our ratings therefore reflect a suite’s recent form, which may not be representative of future performance.

AV-TEST's performance tests time the impact of different anti-virus suites on a number of common desktop tasks, such as downloading and installing applications, launching programs, copying files and visiting popular websites. Once again, AV-TEST publishes percentile scores indicating how much of an impact on performance each program has, and produces an overall score in relation to the average performance of all anti-malware suites. We'll quote this overall score and draw out any particularly interesting performance elements.

Finally, usability tests assess how many false-positive incidents each program produces when tested against 41 popular programs being installed, hundreds of legitimate websites visited, and more than a million examples of already-installed legitimate software and files.

You can see AV-TEST’s raw data on its website and draw your own conclusions if you’d prefer (Excel spreadsheet).

The price of free anti-virus

While you don't have to hand over any cash to use free anti-virus software, you can bet your bottom dollar the company that markets it is making money somehow. This manifests itself in a number of ways. Some products, including Avira and Qihoo, show unobtrusive adverts, while others are paid to bundle software from other companies, which might sneak onto your PC if you’re not paying attention during the installation process.

Any product that's available in paid-for, as well as free, versions, will encourage you to upgrade with varying degrees of persistence, with some including buttons for features that are active only for paid users in their main interface.

Some free AV software, such as Avast, requires you to register, even if you're only using their free incarnation, while others strongly encourage you to do so by providing online monitoring tools that you can use to manage and secure other devices associated with the same account – phones and tablets, for example.

Our reviews detail any obtrusive advertising or promotional features in each free anti-virus suite and, where possible, tell you how to avoid them.

Anti-virus software also reports back to its manufacturer by default when it encounters unknown malicious and even benign files. This means that the databases it relies on are kept constantly up to date, helping to protect all its users and making both paid-for and free versions of the software more accurate.

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