A special tool that blocks the download of Service Pack 2 (SP2) on Windows XP PCs will expire today. Any users who rely on the Automatic Updates feature in Windows to keep their systems up to date will no longer be able to avoid downloading the massive update.

Microsoft released SP2 in August last year. Taken aback by the number of customers who use Automatic Updates, but were not prepared to deal with the service pack, Microsoft provided a way to set a Windows registry key that instructs the system to skip downloading and installing SP2, but still download other critical updates.

Initially the respite was for 120 days. But faced with concerns from IT professionals, Microsoft doubled that to allow more time to prepare for the update. On 12 April the blocking mechanism expires and Automatic Updates and the Windows Update website will deliver SP2 regardless of the block.

"I am ready for XP SP2 now," said Thomas Smith, manager of desktop engineering at a large Houston-based company. Smith blocked the SP2 download on the 5,000 PCs he manages. He has now prepared an additional update that his users need to apply after installing SP2 so they can keep accessing certain required websites, which SP2 blocks, he said.

Still, Smith is not happy with the way Microsoft is "force feeding" him the update. "I am glad that we were able to prepare for it, but next time they need to have a good user round table to discuss this."

SP2 is a major update and has even been compared to a new operating system. The service pack makes many changes to Windows XP to better protect computers against hackers, viruses and other security risks. For example, SP2 includes an improved Windows Firewall, which is turned on by default, and offers memory execution protection to prevent buffer overrun attacks.

As a result of the changes, the service pack can render existing applications inoperable and block access to certain websites. Many businesses want to hold off on installing SP2 and are taking time to test it with their systems.

Microsoft advises consumers to enable Automatic Updates in Windows XP to patch their systems, but recommends businesses use patch management tools such as its Systems Management Server (SMS) and Software Update Services (SUS) or third-party products. As such, Microsoft does not expect many users to be grappling with SP2 once the block is turned off.

"This affects only machines in a handful of enterprise customers' environments," said a Microsoft PR officer. Most customers that used the blocking tool have either installed SP2 or now use special tools for doing patch management, she said.

Microsoft has labelled SP2 a "critical" update and urges all Windows XP users to install it. The company says over 185m copies of SP2 have been downloaded.