Microsoft was scheduled to begin pushing out the third and final service pack for its Windows XP operating system to users on Thursday via its automatic update service.

Though not a major update, Windows XP SP3 offers numerous enhancements over the current, SP2 version of the OS. It includes all updates issued since Windows XP Service Pack 2 was released in 2004, as well as 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.

Additionally, Windows XP SP3 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 reminding Windows XP users who do not wish to receive SP3, which has caused problems on some systems, that they can download a tool called the Windows Service Pack Blocker Kit to prevent it from installing automatically.

Corporate IT managers can also install third-party update management solutions to block unwanted service packs, said Nick MacKechnie, a Microsoft senior technical accounts manager, in a blog post.

After a series of delays, Microsoft released XP SP3, the last major update for the officially retired Windows XP operating system, to broad distribution in May. But the company withheld it from automatic update until it could resolve a number of system and application incompatibilities.

One glitch rendered XP SP3 incompatible with PCs that have Microsoft's Dynamics Retail Management system installed.

Microsoft has since developed a patch that resolves the conflict, but some IT pros might still be wary of Windows XP SP3. Within hours of its public release, many users reported that the update was wreaking havoc on their systems.