Microsoft Corp. announced the release of a long-awaited security update for its Windows XP program on Friday in response to the growing number of security shortcomings in the market-dominant computer operating system.

The free upgrade won't be available to everyone right away, however. Microsoft said the timing will depend on several factors, including customers' Internet usage, location and language as well as the overall demand for the package, dubbed Service Pack 2.

Customers who have computers set to receive updates automatically will begin getting Service Pack 2 within a few days, company spokesman Matt Pilla said Friday. About 100 million customers are expected to receive the automatic updates over the next two months.

Customizing the fixes in 25 languages will take two months, the company said. The update is currently available only in English, and English-language users will get the update as Microsoft distributes it to computer manufacturers, companies and home users through downloads, free CDs and other means.

The upgrade, which Chairman Bill Gates said modifies less than 5 percent of the nearly three-year-old operating system, is designed to make users safer from cyberattacks by sealing entries to viruses, better protecting personal data and fending off spyware.

For most users, the most noticeable change will be a series of new prompts. Users will be asked to give permission for programs to interact with their computers, so there is less chance that their computers will be hit by a virus or inadvertently admit malicious software that can monitor computer activities.

The update automatically turns on a firewall to better guard against attempts to infiltrate personal computers. And it fortifies protections on the Internet Explorer browser and offers tougher policing against e-mail-borne attacks.

A new "Windows Security Center" will help monitor security programs -- including those from outside companies that offer other safeguards, such as anti-virus protection.

Service Pack 2 should be available on compact disc and at http://windowsupdate.microsoft.com by end of the month, Pilla said. New computers will start shipping with it in September or October.

Most users will have to download about 80 megabytes of data for the upgrade. Because the data pack is so big, users are being urged to turn on an automatic update function that will let Microsoft slowly download Service Pack 2 with minimal disruption to normal computer activities.

Microsoft Chairman Bill Gates said the update adds value to Windows XP.Microsoft Chairman Bill Gates said the update adds value to Windows XP.