You don’t have to spend long with Avast Free Antivirus to realize why it’s one of the most popular security tools around.

The program is simple to install; a straightforward interface makes it easy to use; a quick first scan should identify any potential threats on your PC, and this all has minimal impact on your system performance.

These aren’t just subjective opinions, either. Independent testing by AV-Test has found the program to be a good all-rounder, and it was one of AV-Comparative’s “Products of 2012.”

Avast Free has some useful extras, too. A Software Updater alerts you to program updates you’ve missed, for instance, while its Browser Cleanup tool provides a simple way to remove unwanted add-ons from your browsers.

Avast recently added a “Grime Fighter,” which the company says supports 12 additional languages and offers an advanced console for more user control.

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2. Panda Cloud Antivirus Free

Small and simple, Panda Cloud Antivirus is a lightweight tool which provides free real-time antivirus protection, and can be used alongside many other security tools without problems.

It’s the free version of a commercial product, and so has a few missing features. The Pro edition helps to protect you at public wifi networks, for instance, while its “USB Vaccine” function reduces the risk of malware infecting a USB drive.

Panda recently released version 3.0, which includes a bevy of new features for Pro users. However, users of the free software will be able to access a new interface, the ability to schedule scans, and automatic USB vaccinations.

The core antivirus protection remains the same, though. And while that doesn’t deliver the top detection rates, independent testing shows it’s very capable, with the program winning certification in AV-Test’s latest report.

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3. ZoneAlarm Free Antivirus + Firewall

We have one or two concerns about ZoneAlarm Free Antivirus + Firewall, notably that its antivirus database is only updated daily (hourly updates are reserved for the commercial version), which leaves you more exposed to the very latest threats,

The program does provide plenty of functionality, though, with a capable, AV-Test-certified antivirus engine, an easy-to-use firewall and some browsing protection.

It does its best to keep things simple. Open the Preferences dialog, say, and you’ll find only 10 buttons and settings (and most of those you’ll never need to use).

The database update means it wouldn’t be our first choice, but if you’re a fan of ZoneAlarm, or just want a firewall and antivirus tool from the same company, this could be a good choice.

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4. Avira Free Antivirus

Avira Free Antivirus provides two main areas of protection.

A strong antivirus engine (rated highly by independent labs for its file detection rates) constantly monitors your PC, looking for and eliminating threats.

If you choose to install the Avira toolbar then you also get some web tools (an antiphishing tool, ad blocker, social networking protection), although this also replaces your default search engine with an Avira page.