A special tool that blocks the download of Service Pack 2 (SP2) on Windows XP PCs expires today, ending the last SP2 respite for users who rely on Windows' automatic updates feature.

Microsoft has labelled SP2 a "critical" update and urges all Windows XP users to install it. An estimated 185 million have done so, so far.

Yet users are still unhappy with the way that Microsoft is "force feeding" them with the software. "I am glad that we were able to prepare for it, but next time they need to have a good user roundtable to discuss this," said Thomas Smith, manager of desktop engineering at a large US company.

Smith has, however, managed to prepare his systems. "I am ready for XP SP2 now," he said. Smith initially blocked the SP2 download on the 5,000 PCs he oversees but has now produced an additional update that his users need to apply after installing SP2 so they can keep accessing certain required websites that the service pack blocks.

Meanwhile, compatibility issues are keeping law firm ReedSmith from upgrading its 2,600 Windows XP machines. The firm now plans to include SP2 in a refresh of its desktop software starting next year, said David Guilinger, a director in ReedSmith's systems and technology department.

"If we build from scratch, SP2 works fine with our software. If we apply it on top of our existing configurations, we have issues," he said. "Upgrading introduced too many end user prompts and conflicts."

Microsoft released SP2 in August last year. It is a major update and has even been compared to a new operating system. It makes numerous changes to Windows XP to better protect computers against hackers, viruses and other security risks.

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. Over three quarters of Windows XP PCs in US and Canadian businesses have yet to be upgraded, according to a recent survey.

Taken aback by 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 mammoth update. On 12 April (today), the blocking mechanism will expire and Automatic Updates and the Windows Update website will deliver SP2 regardless of the block.