Microsoft will enter new territory Thursday when it begins selling Windows Live OneCare, a PC-maintenance service designed to protect users from viruses, spyware and the loss of computer files.

In the past, the company left most of this heavy lifting to partners in the computer-security market, including Symantec and McAfee. For Microsoft, that didn't always produce the best results.

"Here's what used to happen when there was a big Windows virus: Symantec's stock price would go up, and congressmen were yelling at Microsoft," said John Pescatore, a security analyst with Gartner.

So Microsoft became more aggressive, buying industry vendors such as Giant Company Software of New York and putting together its own service. It will sell for a $50 annual subscription for up to three personal computers.

In doing so, Microsoft has become a direct competitor to Symantec and McAfee, the leaders in consumer PC security. Pescatore predicts the company will have a 30 percent share of the consumer market in three years.

In a sign of what could be growing antagonism between the companies, Symantec sued Microsoft this month, accusing it of abusing a business relationship between the two and misappropriating its technology and trade secrets for use in the upcoming Vista operating system. The lawsuit was not related to OneCare, however.

Dennis Bonsall, director of the OneCare program, said Microsoft still has "a great relationship" with Symantec and McAfee and is bringing some creativity and new ideas to the business.

"What exists out there today for consumers isn't everything they want it to be," he said. "It's not all in one, it's not comprehensive."

OneCare includes anti-virus and anti-spyware features, the ability to back up photos and other files, regular "PC tune-ups" and phone support. Microsoft is launching the service Thursday in the U.S. and plans to roll it out elsewhere within the next year.

Symantec and McAfee are developing similar products. Symantec's service, code-named "Genesis," is slated for a fall release, and McAfee's "Falcon" is expected to launch in the summer. Both are subscription-based.

The price of these services will likely drop in the future as they become bundled with other products, such as high-speed Internet service, Pescatore said. Already, e-mail programs from America Online and Yahoo! will scan and clean out viruses from e-mail attachments.

Kim Peterson: 206-464-2360 or kpeterson@seattletimes.com