The third and last major update for Microsoft's Windows XP operating system will be made available to system builders and business customers next week and to all XP users in late April, according to a widely read tech blog that in the past has accurately predicted release dates for Microsoft products.

Bloggers at Neowin.Net on Tuesday said that Windows XP, Service Pack 3, will be released to original equipment manufacturers and to Microsoft's MSDN and TechNet subscribers on April 21st. XP SP3 will be generally released to Windows XP users through Microsoft's online Windows Update service on April 29, the blog said.

Neowin said it obtained the dates from an internal Microsoft schedule. Microsoft officials could not immediately be reached for comment.

The company last month released to the public a nearly final beta version of Windows XP SP3 called Release Candidate 2 Refresh. Microsoft said the latest beta version added support for the HD Audio high-definition audio format and fixes for some issues with the Windows Update service.

Microsoft to date has said only that it expects to roll out a final version of Windows XP SP3 in the first half of 2008.

The service pack, when final, should offer a number of enhancements over the current version of the OS. It includes all updates issued since Windows XP Service Pack 2 was released in 2004, and some new elements.

Among them: A feature called Network Access Protection that's borrowed from the newer Windows Vista operating system. NAP automatically validates a computer's health, ensuring that it's free of bugs and viruses before allowing it access to a network.

Windows XP SP3 also includes improved "black hole" router detection -- a feature that automatically detects routers that are silently discarding packets. In XP SP3, the feature is turned on by default, according to Microsoft.

Windows XP SP3 also steals a page from Vista's product activation model, meaning that product keys for each copy of the operating system don't need to be entered during setup. The feature should prove popular with corporate IT managers, who often need to oversee hundreds, or even thousands, of operating system installations.

Microsoft is in a bit of a Catch-22 with XP. The more it strengthens the OS, the less reason users have to upgrade to Vista, which by many accounts has failed to catch on with computer users in both the home and office since it debuted last year.

Microsoft recently released Windows Vista Service Pack 1 -- which includes more than 300 hot fixes designed to improve the operating system's speed, security, and stability. Many users, however, have complained that installation of Vista SP1 on their systems caused numerous problems.