Microsoft released SP2 in August last year. 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.

After today the blocking mechanism will expire, and Automatic Updates and the Windows Update Web site will deliver SP2 regardless of the block.

"I am ready for XP SP2 now," says 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 Web sites, which SP2 blocks, he says. 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," he adds.

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 Web sites.

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.