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Steve Chiotakis: Saving the world's computers from viruses and other nasty invasions, it's pretty big business. Starting today, there's new competition from software's biggest name. And Microsoft is giving away its security program. Here's Marketplace senior business correspondent Bob Moon.

Bob Moon: The folks behind the world's most-used -- and, arguably, most cussed at -- computer software are going public with a try-out program that promises to plug the holes in their Windows.

Richard Williams is an industry analyst at Cross Research. He says Microsoft has tried for three years to sell its own brand of anti-virus software, but consumers have overwhelmingly stayed with leading names:

Richard Williams: I think part of that was the perception that Microsoft kind of created the problem, and therefore do you trust them to be the ones to fix it.

Apple has made inroads lampooning Windows' vulnerabilities.

Mac: I'm a Mac.

PC: And I'm a PC -- Ah-choo! Ah-choo!

Mac: Gesundheit -- you OK?

PC: No. I have that virus that's going around.

Williams thinks that may explain why Microsoft has decided to give its program away.

Williams: If it can improve Microsoft's sales of Windows from some of the disaffected types that have gone to Apple, then I think they make a heck of a lot more money off of that, than they would charging for security software.

Williams believes Microsoft's free offering will appeal mostly to those who usually don't buy software anyway, rather than threatening the established anti-virus leaders.

I'm Bob Moon for Marketplace.