Its excellent malware detection, disinfection, and scan speed earned Avira AntiVir Personal the top spot in our August 2009 ranking of free antivirus software. Its interface could be better, though, and using the app means putting up with daily pop-up ads.

In AV-Test.org tests, AntiVir's 98.9 percent overall malware detection rate was the best among the software on our chart (the unranked Panda Cloud Antivirus outperformed it). AntiVir was also tops in proactive-protection tests that use two- and four-week-old signature databases to simulate the detection of new, unknown malware, with rates of 52.7 percent and 45.5 percent, respectively.

The strong performance continued in disinfection tests. AntiVir found and disarmed all of the rootkits and other malware infections tossed at it, though (like all the free antivirus software we tested) it tended to leave some remnants, such as relatively harmless Registry changes, in place.

Avira's program was not just the most thorough tool of the group, but also the fastest. It led in speed tests for both on-demand scans (which you schedule or start manually) and on-access scans (which happen automatically during tasks such as copying files).

If AntiVir's interface were as polished as its malware-fighting ability, it would be a no-brainer recommendation. But its daily pop-up ads for Avira's paid ID-theft protection software could easily annoy many users, and at times its interface feels better suited to advanced users. For example, the program's installer prompts you to select among 'extended threat categories.' Some are obvious, such as games or jokes, but you might be forgiven for not knowing whether to pick 'unusual runtime compression' (listed in Avira's online help as 'Files that have been compressed using an unusual tool and are therefore suspicious').

In a similar vein, its detection pop-ups offer too many choices and don't suggest the option most appropriate for the average user. It checks the 'Deny access' option by default, but that choice would leave the discovered malware sitting on your PC; you would continue to receive warnings until you opted to delete or quarantine the discovery (you can also choose to rename or ignore the file).

Such less-than-friendly default behaviors make Avira AntiVir Personal a better choice for tech-savvy users who know how to muck about in the settings. If you're willing to put up with a somewhat clumsy interface and the recurring pop-up ads, in return you'll enjoy top-notch, free protection against malware. It's not a bad trade-off by any means.

--Erik Larkin