To help you figure out which free antivirus app is right for you, we put packages from all of those companies through their paces. Our testing partner, AV-Test.org of Germany, employed its vast "zoo" of collected malware to test detection rates and scan speed. We then poked and prodded the apps to see which ones made stopping malware an effortless task, and which ones made it feel more like drudgery. For a summary of our findings, see our free antivirus software ranked chart. For our in-depth evaluations, see the individual reviews, linked in this story and in the chart.

Something--But Not Everything--For Nothing

While free antivirus programs give you some value, they don't have everything that a paid security application can offer.

For one thing, you won't have anyone to call if things go haywire, or if you need disinfection help in the event something does sneak past your PC's defenses. Most free apps give support only on online forums, though Avast offers e-mail support (and Microsoft plans to when Security Essentials launches); Avast users can submit online support tickets, too. AVG gives paid phone support, but the $50-per-call fee costs more than most paid antivirus apps.

Do-it-yourselfers can often find good advice at helpful sites like Wilders Security Forums, but even there you shouldn't expect to talk to anyone for help with a free antivirus app. (Unless you can bribe a techie friend, that is.)

Generally, free apps have less-frequent malware-signature updates than paid products do, which can leave a window of opportunity for brand-new baddies to evade detection. Most of the free apps we tried update their signature databases only once daily. Microsoft Security Essentials, however, will also check suspicious samples that don't match a particular installed signature, by running the sample against Microsoft's latest online signatures. And as long as you have an Internet connection, Panda Cloud Antivirus checks everything against Panda's servers, so it will always use the newest signatures. (If you don't have an Internet connection, the Panda program falls back on local caches.)

Some free utilities have fewer scanning options than paid apps from the same company do. For example, Avira's paid antivirus program will scan http traffic to catch Web-borne malware before it hits your hard drive, but the company's free AntiVir Personal version won't. And AVG's paid app ties in to IM programs for additional security, while its AVG 8.5 Free doesn't.