Starting this week, major retail chains began selling Windows XP Service Pack 2 (SP2), the latest version of Microsoft's desktop operating system.

"Microsoft began shipping retail boxes to retailers last week, so they will start showing up on store shelves worldwide throughout the month," said a spokesman with Microsoft's Windows client division.

Among the U.S. retailers offering the Home and/or Professional versions of the product , according to Microsoft, are Amazon.com, Best Buy, Circuit City, CompUSA, Fry's, J&R Computer World, Micro Center, Office Depot, OfficeMax, Target and Wal Mart.

Some of Microsoft's PC partners are just starting now to preload new machines with Windows XP SP2, as opposed to plain-vanilla XP — an expected move which is meeting with mixed reactions from resellers.

Microsoft officials said they had no information to share regarding when leading PC makers will begin bundling the product.

As of September 30, about 40 million copies of Windows XP SP2 had been downloaded by users, according to Microsoft. The vast majority of these copies have been pushed to users via Microsoft's Windows Update/Automatic Update patching systems. Microsoft also has made available to users with slower networking connections a CD version of SP2.

Some customers, especially corporate ones, have postponed their SP2 downloads and deployments, citing the need for more application testing before installing the upgrade, which consists of primarily security-related features and fixes.

Microsoft officials have said they hoped to have distributed 100 million copies of SP2 by the end of October. Microsoft has sold approximately 300 million Windows XP since the product debuted in 2001.

CompUSA is advertising that it will "install Windows XP Service Pack 2 for free" for existing SP2 users. "We'll make your PC safe and secure and save you hours of download time," according to an ad on the CompUSA Web site.

Amazon.com is touting the professional version of XP SP2 on its site. Amazon is offering the product for $189 ($10 off the regular $199 street price) and throwing in a $30 rebate, lowering the final price to $159. (This version is for new customers, not existing XP users looking to upgrade.)

Office Max is advertising both full and upgrade versions of XP SP2.

Office Max is selling the full home version of SP2 for $199.98, or as part of a $224.96 bundle, combined with McAfee anti-spyware and a 128MB QuickiDrive. It is offering the full professional release of XP SP2 for $299.98 (or $324.96 for the aforementioned bundle.) Office Max also is offering the version of SP2 for existing XP home-edition users for $99.98.