Previously codenamed Morro, Microsoft Security Essentials will eventually replace the Windows Live OneCare subscription service, which will cease to be available on 30 June.
The beta version is available only to a limited number of testers in the US, Brazil, Israel and China, and the full product is due for release before the end of 2009.
The product has been getting mixed verdicts in early tests. Some US reviewers concluded that it is up to the task, while others struck a more cautious note on whether it can stand up against rival security suites.
However, the fact that Security Essentials will be free to download should still cause concern among the specialist security vendors.
"Although Microsoft says its intention is to reach the people who currently have no protection, rather than grab market share from security vendors, it cannot fail to take a significant slice of the low-end market in the consumer and small business sectors," said Ovum analyst Graham Titterington.
Security Essentials is designed to offer protection against viruses, spyware and other malicious software, according to Microsoft, including the ability to detect and remove rootkits.
However, the company also said that Security Essentials is a lightweight client that will not overburden system resources, even on older PCs or less powerful systems such as netbooks. It will make use of CPU throttling, and will run scans and signature updates when the PC is idle and use a low-priority thread.
Security Essentials is designed to run on a Windows XP PC with a minimum 500MHz processor and 256MB memory, while Vista and Windows 7 PCs must have a 1GHz processor and 1GB memory.