The vague timetable that Microsoft gave Monday for pushing out Windows XP SP3 to most users got specific Tuesday as the company quietly said it would release the upgrade on Thursday at 10 a.m. PDT.

Microsoft will add Windows XP Service Pack 3 (SP3) to Windows Update at that time, and trigger its automatic download and installation for users who have left the operating system's update service at its default "Automatic (recommended)" setting.

"Updated applicability rules and updated metadata to target the update to Automatic Update channel," read a Microsoft support document revised Tuesday after the company unveiled its monthly security patches . "The Automatic Update change will be published at 10:00 A.M. on July 10."

Monday, a Microsoft manager said only that XP SP3 would be automatically downloaded and installed "shortly," but did not specify a date.

Microsoft delayed the distribution of Windows XP SP3 twice this spring, but the July 10 date seems firm; it was included in the cumulative list of all updates available via Windows Server Update Services (WSUS), the server-side update tool many businesses use, as well as via the client-side Windows Update (WU) and Microsoft Update Web-based services.

A Microsoft spokeswoman, however, was not able to immediately confirm that July 10 will, in fact, be XP SP3's automatic update debut.

In late April, Microsoft postponed the release of XP SP3 because it found a compatibility bug between the operating system and the company's Retail Management System (RMS) point-of-sale software. It also stopped the automatic installation of Windows Vista SP1, which had already been pushed to some users.

A few weeks later, Microsoft added a filter to WU to block AMD-powered PCs susceptible to an "endless reboot" problem from seeing Windows XP SP3 as an available update. In late June, Microsoft crafted a fix and added it to the WU list. Users who had WU set to automatically grab and install updates presumably have this patch already in place.

However, it's not clear how the automatic distribution of XP SP3 will affect users running AMD-equipped machines that haven't been updated with the June fix.

The announcement that Microsoft would release XP SP3 to Automatic Updates came just a week after the company started the operating system on its road to retirement by halting sales of the operating system to retail outlets and barring major computer makers from installing it on most new PCs.

People who want to block the automatic deployment of Windows XP SP3 can either modify the Windows Update settings, or download and use the Windows Service Pack Blocker Tool Kit , which provides several tools for stopping the service pack from reaching client PCs until at least April 2009.