Norton AntiVirus 11
One company's name springs to mind when it comes to antivirus products: Symantec. Norton AntiVirus (NAV) is the 800-pound gorilla of antivirus software, on both the Mac and the PC, and NAV 11 proves that reputation is well deserved. This version features a new, more straightforward interface, and a new Vulnerability Protection feature that scans files and images as you download them to ensure they aren't packing viruses or malware. While it's true that Macs are relatively free of viruses, NAV also scans for Windows viruses, of which there are plenty, which means your Mac won't accidentally pass along dangerous files to your Windows-using friends.
Previous versions of NAV were notorious resource hogs, and version 11 addresses that flaw. Scanning my test Mac's roughly 410,000 files (give or take a GIF here and there) had little to no impact on performance. Surfing the Web, checking e-mail, and listening to music with iTunes were as snappy as I'd expect on my 2.16GHz Core Duo MacBook Pro.
Despite the low impact on your system, there will come a time when you don't want a scheduled scan to take place (perhaps you're in the middle of editing a large video file), and that's where the Snooze feature comes in. Before a scheduled scan is set to run, a notification window appears that lets you delay (or snooze) the scan for anywhere from 15 minutes to a day.
NAV took more than three and a half hours to complete the initial scan of my 170GB of data. This is something to keep in mind, since NAV 11 is configured, out of the box, to scan all volumes that you mount on your Mac, including disk images, iPods, Time Capsule volumes, and removable media. You can easily exclude volumes from autoscanning (a good idea for Time Machine disks) or disable this feature altogether. During my testing the scanning didn't interfere with using the attached volume, nor did it report any false positives.
The Auto-Protect function constantly runs in the background, making sure that your Mac's defenses are not breached. When NAV finds a virus (and it found all the test viruses on my system), an alert window appears in the lower left corner of your screen. NAV will repair any viruses it finds and alert you that it has done so. If it can't repair the virus, it whisks that virus into Quarantine, a special folder on your hard drive, where it can't harm your data. Once a file lands in NAV's quarantine, a few things can happen: NAV can attempt to repair the file and remove it if possible; you can delete the file; or, if it was flagged erroneously, you can restore it to its original location.
Speaking of default settings, NAV's defaults are a bit aggressive. The application automatically scans all attached volumes, as noted above, and it also scans compressed files (.sit, .zip, and so on). NAV actually decompresses and then recompresses files on the fly, which makes such scans slightly more resource intensive. If this default is unchecked, NAV will immediately scan the files when they're uncompressed, so your Mac maintains its level of security.
Macworld's buying advice
Norton AntiVirus 11 is the obvious choice for protecting your Mac in a mixed Windows-Mac environment, and a great option for anyone who is looking for a "set it and forget it" antivirus program. Upgrading from previous versions is worth the price for the scanning improvements alone.
[Scott McNulty is a full-time geek living in Philadelphia. He is the lead blogger at The Unofficial Apple Weblog and cohosts the cooking podcast Fork You.]