Formerly known by the code name "Morro", Microsoft Security Essentials (MSE) replaces the commercially unsuccessful Windows Live OneCare subscription security product for up to three PCs.
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MSE is a trimmed down version of its predecessor and is aimed at providing basic security protection to users of Windows to improve the overall security standing of Microsoft's operating system.
Thousands of registered Windows users have downloaded a beta version of MSE since June.
There is no firewall, back-up capabilities or spam filters, but MSE still provides protection from viruses, Trojans, spyware and rootkits.
According to Microsoft, MSE consumes minimal CPU and memory resources and will not interfere with normal computing tasks.
"Consumers have told us that they want the protection of real-time security software, but we know that too many are either unwilling or unable to pay for it, so they remain unprotected," said Amy Barzdukas, general manager for consumer security at Microsoft.
MSE provides real-time malware protection by linking to Microsoft's Dynamic Signature Service, which keeps users protected with the latest virus definitions before they are officially released.
Commercial security software suppliers claim they are not concerned by Microsoft's move because free software does not have the support that the more important business market requires.
But more than half of UK small and medium businesses which use security software rely on free products, a survey has revealed.