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Note: This review addresses version 4.5 of the software.

Unlike most of the other pieces of free antivirus software that we tested recently, PC Tools Threatfire is not a stand-alone antivirus program. Instead, Threatfire supplements your existing security app with highly effective behavioral analysis that can stop malware based solely on what the file tries to do on your PC.

Behavioral detection attempts to thwart the successful hacker tactic of churning out ever greater numbers of malware variants to stay one step ahead of traditional antivirus signature databases (which the majority of security programs use). Other programs employ behavioral or heuristic techniques, too (the latter looks for partial matches with known malware). But behavioral detection like the approach that PC Tools has implemented here can be particularly tricky, as it's prone to accidentally flagging harmless software.

In's behavioral-detection tests, most apps have only about a 30 percent to 60 percent detection rate. Not so for Threatfire: It warned about every single one of the 15 malware samples used, and it blocked all but one of them. What's more, this nimble malware nabber didn't put up any false-positive warnings. It's hard not to be impressed with such stellar performance.

While running more than one regular antivirus app at the same time can cause major problems, Threatfire should run smoothly alongside existing security tools, including any of the free antivirus software we ranked for our recent story. Since it detects only programs that attempt to run, you'll still want a standard antivirus program to perform regular scans and to check files that write to your hard drive, before they get a chance to execute.

Threatfire is simple to use and defaults to an appropriate medium level of sensitivity, but we suggest turning on the option in Settings, Quarantine to create a system restore point automatically before quarantining anything. As for extras, a nice system-activity monitor provides extensive technical details about all currently running programs, and a mostly just-for-show threat monitor maps global malware outbreaks.

Though Threatfire is designed to run with other apps, and we had no problems in our tests, it has conflicted with AVG in the past. That aside, we recommend Threatfire as a strong extra layer of defense.

--Erik Larkin

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