Note: Our best free antivirus round-up has been fully updated. This feature was first published in August 2014.
Staying loyal to the trusty built-in Windows Defender keeps you safe to a certain extent, but with independent testing showing it misses at least one threat in 10, you really should look elsewhere.
And while your Android device has stronger core security, it's still vulnerable to the new crop of malicious apps, and you're going to need some help to stay safe.
Luckily paying through the nose for this type of defence is no longer the status quo and there are scores of different programs offering protection that is more than adequate.
So if you no longer want to pay for the privilege of protecting your valuable piece of hardware, read on - we've picked out four of the best free antivirus engines, two top Android apps and two online detection services, all delivering great security at no cost.
- Also check out: Best antivirus: 10 antivirus packages reviewed and rated
1. 360 Total Security
If you're looking for a standalone antivirus package then 360 Total Security might seem a little overweight. It provides antiphishing support, online shopping protection, network threat blocking, hard drive clean-up tools, a Windows update checker, and more.
This feature overload does make for a relatively complex interface. It's not always easy to operate, and if you run into problems there's no significant documentation to help.
Still, what makes the suite interesting is that it uses no less than five antivirus engines. Install it and you can be protected by Bitdefender technology (an excellent commercial engine), Avira (probably the best of the free offerings), and three further engines of Qihoo's own.
The default settings aren't necessarily the best, and you'll probably have to spend a while making sure it's configured to suit your needs.
We found the multi-engine approach can affect performance, too, with 360 Total Security taking anything up to twice as long to scan our system as some of the competition, as well as returning more false positives.
Surprisingly, however, all this bulk didn't weigh down our PC in normal use, with 360 Total Security having a minimal effect on system speed.
360 Total Security is far from perfect, and if you're looking for a simple antivirus engine to run alongside other security tools then it'll probably be too much. But if detection rates are your top priority then the package is a real contender.
2. Avast Free Antivirus
While free antivirus software won't cost you anything to download, you might sometimes find you'll pay in other ways. Install Avast Free Antivirus and it'll also equip your PC with Google toolbar, unless you're paying attention and clear the relevant checkbox.
Scan your system for malware and by default Avast Free Antivirus also checks for junk files, unnecessary apps and other performance issues, although it won't fix any of these unless you buy the company's PC Optimizer.
Fortunately you don't have to live with this. Avast Free Antivirus has an excellent interface – it's clean, clear and configurable – and in just a few clicks you can set it up to avoid the time-wasting performance scan.
The testing labs currently give Avast good, but not exceptional results; typically the company outperforms AVG, but can't match the power of Avira.
Still, the package does rate highly when dealing with zero-day threats, and in our experience gives very few false positives.
Crucially, it's also better than most at blocking malicious URLs, which means you're less likely to encounter malware in the first place.
Useful extras include checks for network security holes (like a router still using the default password), while the Software Update highlights missing software patches and installs them with a click.
Avast Free Antivirus may not quite offer the best protection, then, but its interface is one of the best around, and experienced users will appreciate the program's extreme configurability.
3. Avira Free Antivirus
Some security companies spend a great deal of time and effort on interface design, but Avira isn't quite so concerned. Sure, Avira Free Antivirus adds a graphical launcher to your system tray, but the core program looks plain, ordinary, and a little dated.
Fortunately Avira scores where it matters. The company seems much more interested in substance than style, and the end result is one of the best security freebies around.
The program is loved by the testing labs. Whether you check out AV-Test, AV-Comparatives or VB100 reports, Avira technology almost always scores very highly. And that's not only in relation to free software. AV-Comparative's July 2015 Real-World Protection Test ranked the program fourth out of 20 mostly commercial contenders, with its 99.7% protection rate comparable with the highly-rated Bitdefender (99.8%) and Kaspersky (99.7%). That's a typical score, not a fluke – sometimes Avira tops the list.
If you're looking for an easy life then you can leave the program alone, and it'll do its work almost entirely automatically. But expert users get plenty of fine control – there are options to protect the Hosts file, block autorun, scan archives, monitor network drives, set the file types to be scanned, even password protect your Avira installation so others can't mess with your settings.
Avira Free Antivirus has a distinct shortage of bonus extras, which might be an issue for some. The "Firewall" options add little (they configure the standard Windows firewall, rather than adding anything new), and even basic web protection requires a separate browser extension.
Still, for us, Avira's stripped-back design is a plus, as it makes it easier to add other security tools without conflicts. Factor in its many configuration options and Avira is a smart choice for more experienced users.
4. Panda Free Antivirus
The subdued blue-green interface of Panda Free Antivirus isn't what we'd call visually impressive, but otherwise works very well, making it easy to find and access the program's features.
There's no doubt about its accuracy, either. AV-Comparatives' monthly Real-World Protection Test has seen Panda in the top two for months now, trampling all over the commercial competition. And independent testing shows it manages this while raising very few false alarms.
URL filtering is another significant plus, with Panda doing an above-average job of blocking access to malicious sites.
Bonus extras include USB Protection to keep your USB keys safe from some malware, while Process Monitor is a Task Manager-like tool, displaying running processes, their open HTTP connections, and highlighting likely dangers.
If you're feeling picky, and spend long enough exploring the program, you might find one or two minor issues. Scanning speeds were marginally below average in our tests, for instance, and the program isn't as configurable as some of the competition.
It's hard to complain about a package which gives you market-leading protection at zero cost, though. If you're looking for something simple and reliable, which you can install and just leave to do its job, then Panda Free Antivirus is a great choice.
5. Avira Antivirus Security 4.2
Selecting the best free Android antivirus app is difficult at the moment. It's a very competitive market, and the independent testing labs consistently show there's very little between the leading products.
Avira Antivirus Security earns our vote for its consistency. It's a reliable engine, typically amongst the top-rated apps at AV-Test and AV Comparatives (and it's 4.4 star rated at Google Play), and Avira's excellent record with PC security software makes us think you'll be able to trust the product over the long term.
There are plenty of bonus features, too. Anti-theft tools can locate a lost phone, trigger a siren, or remotely lock or wipe a device, and privacy ratings give you an indication of how apps are handling your data, plus you also get identity protection, app locking, call filtering and more.
Browsing protection, hourly updates and tech support are reserved for the commercial Pro version only, but Avira Antivirus Security provides the core essentials you need and does a great job of keeping you safe.
6. 360 Security - Antivirus FREE
360 Security - Antivirus FREE is one of the most popular antivirus apps around, and the name alone gives you one reason why: it really is free. Okay, you get ads, but there are no "upgrade" buttons or limitations to the protection you get.
There's a lengthy list of extras, too. The optimisation features alone - junk file cleaning, memory booster, power saver - are more capable and effective than many standalone apps.
We're particularly interested in the antivirus, and 360 Security - Antivirus FREE delivers there too. It's an extremely accurate engine which doesn't generate too much traffic, significantly drain the battery, or otherwise get in the way of your regular activities.
The system also monitors apps as they're installed, blocking malware before it can do any harm, and there's a good set of bonus features: anti-theft, call and SMS filtering, an app locker and more.
Installing a good antivirus engine will keep you safe from most threats, but no product offers a 100% detection guarantee. Even if a download hasn't raised an alert, you might sometimes wonder if it's safe.
Upload a suspect file to VirusTotal and within seconds you'll see the scanning results from 56 leading antivirus packages, along with an in-depth analysis of the file, and even what it does when run on your computer (the modules it needs, the files it reads and writes, and more).
One problem with this multi-product approach is you'll regularly see false positives, where most of the engines say a file is safe, but a few flag it as dangerous. If only a small number of packages raise alerts (maybe five or less), and the most trusted engines say it's safe, we might run the file anyway - but that does increase your risk of infection. Wait a day or two and try scanning the file again, if you're concerned.
8. Metascan Online
VirusTotal is the king of the online virus scanners, but there are some competitors around, andMetascan Online is one of the best.
The core idea is much the same. Point Metascan Online at a file, and it'll be uploaded and scanned by a host of antivirus packages, with their verdicts listed in a detailed report.
MetaScan Online uses less engines than VirusTotal - 43 as opposed to 56 - but that's not necessarily a bad thing. Dropping the weaker products should mean fewer false positives, and we also found reports were generated far more quickly.
MetaScan Online can also scan marginally larger files than VirusTotal (140MB compared to 128MB), and has a neat Scan History page to show how a file's detection rate has changed over time. But balancing that, it doesn't provide any behavioural information, and malware doesn't normally arrive in such large files anyway.
Overall, VirusTotal is still our online favourite, but MetaScan Online has plus points too. Give it a try and see how the site works for you.