Windows XP SP 2... ready at last!

Microsoft has wrapped up development on a long-awaited security update to Windows XP, paving the way for businesses and consumers to upgrade in the coming days and months.

The company said it has released Windows XP Service Pack 2 to manufacturing, following a series of delays. Microsoft will make the free update available via download and via CD, but it is recommending that customers turn on Windows' automatic upgrade feature and get the update that way.

The update should be distributed to approximately 100 million PCs through automatic updates during the next two months, Microsoft said. Customers who choose to download manually will be able to do so by the end of August.

Microsoft Chairman Bill Gates said in a statement: "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."

The software adds a new 'security center' that is intended to provide a beefed-up firewall as well as easy ways to tell whether a PC is updated and protected against viruses.

Microsoft said it is working with computer manufacturers to get them to start using SP2 as soon as possible. The Redmond, Wash., company also said it plans to create 25 localized versions of the update in the coming months.

The timing of when customers will get the software via the automatic update feature depends on several factors, Microsoft said, including customers' location, how often they're online, their connection speed and what language version of Windows they use. Those using the English version of Windows will start receiving updates in the next few days, Microsoft said.

In addition to the security center and firewall, SP2 adds a pop-up ad blocker in Internet Explorer and updated support for Wi-Fi and Bluetooth wireless technologies.

The company had planned internally to release the software earlier this week but pushed it out to fix some unspecified quality issues.

Gates has described SP2 as the most extensive free update to Windows ever, and executives have acknowledged that work on the update has delayed other projects, including Longhorn, the next major version of Windows.

In addition to making the software available via automatic update, Microsoft will allow IT managers to download an upgrade that companies can use to update their machines. That process will start next week.

Microsoft said the release of SP2 also brings with it major improvements to two specialized versions of Windows XP: the Tablet PC and Media Center editions. The new tablet operating system, for example, includes improved handwriting recognition and other enhancements, according to the software giant. The automatic update feature will sort out who needs what.

Microsoft product manager Matt Pilla said: "If you are running Tablet PC, you get the Tablet PC update. If you are running Media Center, you get Media Center."

As a result, the size of the SP2 download varies, though Pilla said the typical Windows XP home user with automatic update turned on will have an 80MB download. Microsoft has software that allows both broadband and dial-up users to get the update in chunks over time.

Although there have been concerns that the new security features will 'break' various Windows programs, Pilla said he expects the update to be painless for most people.

"The great majority of consumer applications - it's going to be a great experience on those," he said. The biggest change, he said, will be that the operating system alerts customers when a program is trying to, say, open a port to the internet. "I think there will be some adjustment on the part of consumers as we provide more information."

Ina Fried writes for CNET News.com