Microsoft’s recent announcement that it will distribute free antivirus software was unexpected but in some ways it’s just an expansion of efforts by Microsoft that have been ongoing for years. In 2006 Microsoft began including the “Malicious Software Removal Tool” in the monthly automatic updates for every Windows computer. Every month, your computer is checked for dozens of specific bits of malware and cleaned if necessary.

In the last couple of months, Microsoft has targeted the fake security programs that are prevalent now – here’s my writeup about one variation that presents realistic, professional-looking warnings of viruses and insistently tries to convince you to surrender a credit card number for $49.95 of useless software.

The removal tool runs automatically and silently. It does not need to be run manually, although it’s always possible to visit the Microsoft web site for the tool and download it as part of responding to any possible virus or malware attack. It’s not a substitute for an antivirus program – it doesn’t run continuously watching for threats and it is not directed at all of the malware out there, just a specific set of programs that represent the worst of the current malicious software.

Anonymous data is sent  back to Microsoft so it can track the effectiveness of the tool. In November, the removal tool cleaned bad stuff from almost a million computers in nine days, and in December, it removed the prevalent “Antivirus 2009” malware from 400,000 computers.

As always, the bad guys are getting better at imitating legitimate programs. Read carefully, be skeptical, and be careful out there!