Freeware antivirus Avast 5 debuts today with several new features, but longtime fans are most likely to notice that the old interface has gone to wherever interfaces go when they die. Along with the new interface, Avast Free, Avast Professional, and the new Avast Internet Security introduce an overhauled feature set that keeps the suite highly competitive. Arguably, the free version provides the most complete free antivirus on the market.

Before detailing the new features, the importance of this interface change can't be understated. Gone is the music player default look, which was skinnable but confusing. In its place is a sleek user interface that new users should find far more manageable. The gray and orange color scheme stands out well on the screen, and the tab-based navigation on the left makes it much easier than before to navigate between features. Highlighted with the familiar security colors of green for safe and red for dangerous, the Summary tab gives up-to-date info on shield status, auto-updates, virus definitions, the program version, and whether the new silent/gaming mode is on. There's also an unobtrusive ad to Avast Internet Security.

The Summary tab contains a second submenu, Statistics. If you're curious to see how Avast's shields have been performing against threats, here's where you can get your math geek on. For each shield, it tells you how many files were scanned and when, and presents the data in a concise graph.

The scans live in the second tab, where you can choose and adjust six default scan types plus a custom scan option. What's useful about Avast's layout here is that you can adjust all Avast-related scans from this tab. This includes the expected Quick and Full scans, but also encompasses the Removable media scan, Folder scan, Scans initiated from Windows Explorer, and the Screen Saver scan. Once you initiate a scan, you're not locked into that pane. Exploring the program interface while a scan runs doesn't kill the scan.

You can also schedule a boot-time scan and access scan logs from the scan tab. While running a scan, Avast will tell you not only how long the scan has taken and how many files have been examined, but also how much data has been tested and how fast it's being tested. As with the summary graphs, there's a lot of data that Avast exposes.

The new, clean look of Avast. (Credit: Screenshot by Seth Rosenblatt/CNET)

The Real-Time Shields live in the third tab, and again the clean interface comes into play here as navigating what could be a mess of options and tweaks is instead dead simple. Click on a shield to reveal a real-time chart of what it's been defending you against, with a Stop button and settings options at the top of the window. Another button at the top takes you to the advanced settings for that shield, and links at the bottom expose the shield's history as a graph and export a log file.

The last tab, Maintenance, contains the virus chest and manual update buttons. On the top right of the interface live the Help Center and the Settings, from which you can get much more granular control of Avast. This includes everything from toggling the system tray icon, to managing updates, to password-protecting Avast access. This is also where you can uninvolve your anonymously submitted data from Community IQ, the Avast crowd-sourced behavioral detection engine. The new Avast interface is also Aero-friendly, with Explorer-style backward and forward navigation buttons in case you can't remember where you tweaked a particular setting. If the interface turned you off before, Avast 5 is worth another look just to see how much it's improved.

The basic free version, formerly known as the Home Edition, includes an attractive and obviously affordable set of features. The antivirus, antispyware, and heuristics engines form a security core that also includes multiple real-time shields. Mail and file system shields join the preexisting behavior, network, instant messaging, peer-to-peer, and Web shields. Other new features include a silent-gaming mode and an "intelligent scanner" that only looks at changed files after establishing a baseline.

The old look of Avast 4.8 and its settings window. (Credit: Screenshot by Seth Rosenblatt/CNET)

The behavioral shield is a common-sense feature, as security software publishers leverage their large user bases to detect threats early and warn others. Avast's user base is interesting in that it's heavily involved in the support of the software, and the company claims that its mostly volunteer-run support forums see 100,000 daily visitors. This isn't surprising, though, given that Avast passed the 100-million-user mark at the end of 2009.

There are several features that are only available in both the Professional and Internet Security versions. The Pro version, which comes with a single-computer license for $39.95, offers a script shield and a sandbox for isolating and testing suspected threats, without having to worry about infection. The Internet Security version gives users three licenses for $59.95, and includes a firewall and antispam measures along with the sandbox and script shield.

Efficacy testing of antivirus programs is becoming trickier as threats mutate beyond what the tests have been designed to check, but some testing is better than none. In the Whole Product Dynamic Test by independent efficacy tester AV-Comparitives in December 2009, which tested a beta version of Avast 5, the program earned an Advanced rating. Although it shared that score with six others, including Eset and the free programs from Microsoft and Avira, only two programs scored higher: Norton and Kaspersky.

In November 2009, Avast 4.8 and Microsoft Security Essentials were the only freeware to score Advanced+ in the Retrospective/Proactive Test done by AV Comparatives. Avast 4.8 was also the only program that tested faster than Norton AntiVirus 2010, and was tied with Microsoft for second place in fewest false positives detected. The detection score was also high, at 98 percent. That's worse than Avira, but Avast noted far fewer false positives. Avast 5 Free lacks features in the paid upgrade including antispam measures, a testing sandbox, a Script shield, and a firewall. Even without those, Avast Free 5 probably the strongest, free antivirus currently available.

Avast 5 exposes a lot of data for people to obsess over. (Credit: Screenshot by Seth Rosenblatt/CNET)

CNET Labs is currently performing its own tests, and the results will be added here when they're available. In an empirical real-world test, though, the Quick scan took 13 minutes, 52 seconds, while a Full scan ran about 90 minutes.

Users should also note that along with offering an opt-in to Google Chrome when installing Avast, the free version was also chosen last week to be part of the Google Pack in Europe.

Though it's clear that the paid versions of Avast represent a good value, albeit not the best value, the efficacy, feature set, and ease of use of Avast Free make it a must-have security suite.