This low-cost antivirus tool performed the best in our heuristics tests and caught the widest range of malware.

BitDefender's Malware Detection: Top-Notch

We have both good and bad news about the ongoing war against computer viruses. The good news: All the antivirus products we tested for this article were 100 percent successful at identifying and blocking recognized security threats. The bad news: Such utilities still can't completely protect you from new threats--and there are plenty of those around.

AV-Test, the German security firm with which PC World partnered for this story, says that 70 to 100 new threats are discovered each day. Though many of them are variants of existing threats, waiting even a few hours for your antivirus software vendor to release fixes for them exposes your computer and others to harmful infection. Plus, viruses aren't the only problem. Virus writers are also sending worms--which don't need a host file in order to spread--and other destructive programs such as Trojan horses to users as e-mail attachments.

"The Bagle author likes to do this," says senior security researcher Joe Stewart of LURHQ, a company that provides security consulting and managed security services. Because of such dangers, it's important that your antivirus application be able to recognize and remove not only viruses but other types of threats as well.

(Read story: "The New Virus Fighters.")