Note: This review addresses v5.1 of the software.

Avast Pro Antivirus 5 ranks third in our 2011 roundup of paid antivirus products. When PCWorld last looked at Avast's paid antivirus offering, our reviewer was unimpressed with its interface and its malware detection capabilities. Avast Antivirus Pro 5 is a definite improvement, thanks to a slick new interface and some useful additional features. But middling detection capabilities relative to other paid antivirus software keep it from claiming a higher ranking.

Avast's installation process is quick and painless, and I had to click through only a few screens before it started installing. The main screen has four main tabs running along the left edge--one each for the summary screen, scans, real-time shields, and maintenance tools. I found the main screen attractive and well laid out, but the scan results screen needs a little work: When malware is detected, it doesn't tell you what the malware does, or why it's dangerous.

The settings screens were good, but not outstanding. You'll see a lot of settings for you to change. Even so, you'll find some limitations--no option to choose what happens to malware when it's detected, for example, and little description of what the individual settings do.

Avast Pro Antivirus 5 did a reasonable job at detecting and removing malware overall, but it did have weak points. In traditional scanner-based malware detection tests, Pro Antivirus 5 detected 94.8 percent of samples--somewhat below average for this test (96.2 percent) and behind the top scorers, which detected over 99 percent of samples. These tests rely heavily on malware signature files, so it's a good indicator of how well a product can detect known malware.

In tests that judge how well antivirus products can detect and stop brand-new malware, Avast Pro Antivirus was successful in completely blocking malware attacks 80 percent of the time, which is about average, and it partially blocked an additional 4 percent of attacks. No product we tested was able to block every attack, but the top scorer in this test fully blocked 96 percent of attacks.

Avast's middling performance held true in tests measuring how well antivirus software detects and removes malware infections: It detected all infections--as did all other products we tested--and removed active components in 70 percent of the cases. This places Avast right around average in this test, though most products score in the 70 percent to 80 percent range. It was able to remove all components of 30 percent of the infections, which places it below average (the top performer in this test removed all traces of infections 70 percent of the time).

One feature I particularly liked was Avast's boot scanner, a tool that will scan your PC for malware upon startup--before Windows even loads. The logic behind this is that it will catch malware on your system before anything runs. According to Avast, the program will let you scan and clean an infected PC before the malware does any damage.

Avast Pro Antivirus is speedy as well: It had the fastest scan speeds of every paid antivirus product we looked at, and its impact on overall system performance was relatively low. It completed an on-demand scan of 4.5GB of data in 90 seconds--tying as the fastest finisher in this test. It also lead the way in on-access scans, which kick off when files are opened or saved to disk: It completed the test in 3 minutes, 40 seconds--20 seconds ahead of its closest competitor.

If you're looking for an antivirus program that's quick and easy to use, Avast Pro Antivirus 5 should be right up your alley. But if you're looking for the absolute best antivirus protection, consider Norton Antivirus 2011 instead.

--Nick Mediati