SEATTLE — Security software makers, the 800-pound gorilla has landed.

Microsoft Corp. was to announce today that it is releasing software that aims to better protect people who use its Windows operating system from Internet attacks. The move pits the world's largest software maker head-to-head with longtime business partners Symantec Corp., McAfee Inc. and others.

Windows Live OneCare, which will protect up to three computers for $49.95 per year, marks the latest step in Microsoft's effort over the years to make its operating system less vulnerable to crippling Internet attacks.

Windows, which runs on the vast majority of personal computers, has been a near-constant target of worms, viruses and other attacks, hurting countless users and forcing Microsoft to invest heavily in patching vulnerabilities and improving flaws.

The official release of the OneCare product comes after months of public testing. Redmond-based Microsoft has previously said that its main focus for OneCare was the 70 percent of computer users who, according to Microsoft estimates, have no additional protection at all.

But last week, Ryan Hamlin, general manager for the OneCare product, said the company also hopes to snag existing Symantec and McAfee customers.

"We'd love for those customers to use our product, and encourage them to, but there's also 70 percent that don't use anybody," he said.

Microsoft is hoping to gain an edge against Symantec and others by also including tools in OneCare to make computers run more smoothly and help people back up data.

McAfee said yesterday that it was preparing to release a new security service, code-named Falcon, this summer.

A spokesman for Symantec, maker of the popular Norton products, said no one was available to comment on the OneCare competition.

Hamlin said he expects the product to be profitable for Microsoft.

He said the company doesn't have any current plans to bundle OneCare into the Windows operating system, as it has done with products such as its Internet browser and music and video player.

But he said the company was looking at ways to distribute the product through computer makers or Internet service providers, as many competing security software makers have done.

The OneCare release also comes on the heels of a federal lawsuit Symantec filed against Microsoft over a separate matter.

The lawsuit, filed in federal court in Seattle, accuses Microsoft of misappropriating Symantec's intellectual property and breach of contract.

The dispute is over a technology that allows operating systems to handle large amounts of data.

Hamlin said Microsoft believes it acted appropriately.