Sophos unveiled free antivirus software today that Macintosh users can install to detect and block malware that targets computers running the Mac operating system.

Sophos AV software detects a Mac virus.

Sophos Anti-Virus Home Edition for Mac also detects malware written for Windows that Mac users can spread via USB drives and e-mail, Chet Wisniewski, a senior security adviser at Sophos, told CNET.

The software is a noncommercial version of Sophos Anti-Virus 7.2 for Mac, targeted at home users and modified so it is easier to install and use, he said.

The free Mac antivirus software from Sophos, which also launched a Mac support forum, will compete with ClamXav, Avast, and PCTools' iAntiVirus. Sophos has more information about Mac threats in this blog post.

While the vast majority of malware is written for the Windows platform, Mac threats do crop up. Last week, the Boonana Trojan horse surfaced. And in an update to Snow Leopard in June, Apple added software to protect Macs from a Trojan horse that was being distributed by attackers disguised as iPhoto.

Updated 8:57 a.m. PDT and 9:30 a.m. PDT: Links to Sophos Anti-Virus Home Edition for Mac and to the Sophos blog post have been added. Also, the image has been replaced.