Security firm Sophos recently examined 100,000 Mac computers that had downloaded the company's free anti-virus software, and found that 20 percent had been hosting at least one type of Windows malware.

The top Windows malware the security firm found on Macs was as follows:

1. Mal/Bredo 12.2%
2. Mal/Phish 7.4%
3. Mal/FakeAV 3.8%
4. Troj/ObfJS 3.6%
5. Mal/ASFDldr 3.3%
6. Troj/Invo 3.0%
7. Troj/Wimad 2.6%
8. Mal/Iframe 1.5%
9. Mal/JavaGen 1.4%
10.Other 61.2%

"Some of the malware found dated back to 2007, the company said, indicating that many Mac users rarely, if ever, run regular scans for bad software," writes The Washington Post's Hayley Tsukayama.

"When stored on a Mac, Windows malware is inactive and can't do any harm, unless that computer has Windows installed as a secondary OS," notes Computerworld's Lucian Constantin. "However, such malicious files can still be transferred unknowingly by Mac users to Windows machines via file sharing, USB memory sticks, external hard disk drives and other removable media devices."

"More disturbingly, Sophos's analysis also shows that 2.7 percent (one in 36) of Macs which downloaded the free anti-virus product were found to be infected by Mac OS X malware," notes Sophos' Graham Cluley.

"Of these systems, the majority were infected with the recent Flashback malware, but others have also been found including FakeAV (MacDefender and its variants), and RSPlug (the DNSChanger botnet)," writes CNET's Topher Kessler.