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, called 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 3-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.
"Service Pack 2 is a significant step in delivering on our goal to help customers make their PCs better isolated and more resilient in the face of increasingly sophisticated attacks," Gates said.
For regular 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 they 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 an array of security programs - including those from outside companies that offer other safeguards, such as antivirus protection.
Service Pack 2 should be available on compact disc and at http://windowsupdate.microsoft.com by the 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 onto your computer with minimal disruption to normal computer activities.
"It's a smart download," Pilla said. "We only download to your computer the bits your PC needs."
Microsoft wouldn't say how long the downloads will take, saying it depends on many factors.
But on average, Pilla said, it should take about 60 to 90 minutes for users with a broadband connection to download the upgrade, as people will likely be Web surfing and doing other things that take online priority. He said the download could take users with a dial-up connection a few days to get.