Microsoft unveiled on Thursday plans to launch a computer subscription service that would include anti-virus and security updates for personal computers.

The Redmond, Washington-based software giant said it would initially distribute the service, called "Windows OneCare," among its employees this week and would offer a trial, or beta, service for PCs running Windows later this year. Users would pay a yearly subscription fee.

In addition to providing anti-virus, anti-spyware and firewall protection, Windows OneCare will include tools to keep PCs maintained for optimal performance and back up key data.

Microsoft did not say how much it would charge for the service, which had been expected for a while by the security software community.

Microsoft acquired anti-virus technology from GeCAD Software Srl., a Romanian software company, in 2003 to develop its own software. A major security-focused update to Windows XP, called Service Pack 2, was delivered last year.

Such efforts have hit the shares of major security software vendors. McAfee Inc.'s shares are down 13 percent so far this year while Symantec Corp.'s stock has declined 28 percent in the same period.

Nearly all of the major anti-virus security programs are designed to be updated regularly over the Internet for a monthly or yearly fee, so that they can cope with new virus threats as they appear.

Apple Computer, which recently launched a major update to its Macintosh computer operating system, has long offered an anti-virus subscription service for its users.