Key features:

Silent detection mode
Multiple protection engines
Windows XP/Vista/7/8/8.1/10
Requires 1.6GHz processor, 512MB RAM, 1GB free disk space

The somewhat confusingly named Qihoo (pronounced chi-hoo) 360 Total Security – not a million miles from the name of a popular product in Symantec’s Norton range – is a relative newcomer to the free anti-virus market, but it’s equipped with an impressive set of tools and features.

Unusually for a free anti-virus product, Qihoo 360 Total Security uses its own scanning engines alongside up to two standard anti-virus engines to provide the most accurate possible results. It combines the Avira and Bitdefender engines, although you have the option of choosing not to install the latter engine at setup time. If you want to use Avast as well, you’ll be given the option of installing it later.

Bearing this in mind, it perhaps isn’t surprising that its virus detection performance in AV-TEST’s spring tests was flawless, picking up 100% of both live real-world exposures and all malicious files in a reference set. It didn’t misidentify any legitimate software as malicious, either – although, oddly, it threw up warnings during specific parts of the installation and operation of three benign programs, which is a minor blip in generally excellent performance.

Its impact on system performance is a slight weak point that may put off users with underpowered PCs: AV-TEST found a notable impact on the speed of software installs and file-copy speeds. Nonetheless, Qihoo’s perfect detection score puts it in a rare and impressive category for either free or paid-for anti-virus.

The Qihoo 360 Total Security client is a plain-looking blue-and-white affair – with optional skins on offer. It includes a number of system optimisation features that you may or may not want in its default Full Check scan, such as boot-time speed up and junk file clearing scans. However, you can just switch to the client’s Virus Scan tab if all you want to know is whether or not there’s malware on your PC.

The Virus Scan tab lets you run a quick, full or custom scan, and is also where you go to enable and disable Qihoo’s various scanning modules. It’s here that we were first given the option to download and install the Avast engine. The full range of engines used for this scan are the 360 Cloud Engine, System Repair Engine, QVMII AI Engine (all built in-house by Qihoo), Bitdefender and Avira.

If it encounters any files that are new to it, even if they’re not malicious, it will also ask permission to upload them for analysis and inclusion in its directory. Other buttons on the Virus Scan tab let you see, delete or restore anything that’s been quarantined, and edit your Trust List – a list of files, directories or URLs that you wish to exempt from scanning for any reason.

Other tabs provide access to Qihoo’s Speedup system optimisation tools, while a general Tool Box tab is packed with various Qihoo utilities, including a registry cleaner, a sandbox to safely run potentially risky software and a vulnerability scanner.

Meanwhile, if you want to replace the Windows Firewall, Qihoo has partnered with well-regarded software firewall vendor GlassWire to let you install GlassWire Firewall as an integrated component. While most of these bells and whistles aren’t particularly important to an anti-virus suite, it’s nonetheless impressive to see so many features packed into a free offering.

Qihoo 360’s settings offer up a few extra customisation options, allowing you to install browser plugins to warn about potentially dangerous sites and files when you encounter them on the web, scan compressed files and USB disks, block potentially unwanted programs from being downloaded – along with more desirable software – and disable the upload for analysis feature, among others.

A Notification Area icon lets you put the program into silent mode, so it doesn’t bother you when gaming or watching a film. It’s worth noting that Qihoo is ad-supported, but this is incredibly unobtrusive: just a couple of “recommended free games” in a bar at the bottom of the toolbox.

Qihoo 360 Total Security is still something of a newcomer to the anti-virus market, but we were pleasantly surprised by its clear interface, huge range of features, and its ability to use multiple scanning engines for optimal results, giving it a perfect protection score in AV-TEST’s spring tests.

Perhaps not surprisingly for a product with so many engines and features, it has a heavier system load than some of its rivals. But if you have a powerful PC then Qihoo, with all its engines activated, provides the most comprehensive free protection around.

Users with less powerful computers should opt for Avira instead.