Microsoft is alerting customers that it will soon start delivering Windows XP Service Pack 2 to all customers using Automatic Update, whether they want it or not.

In response to requests from businesses, Microsoft last year released a tool that allowed companies to continue using the automatic update feature but temporarily block the security-oriented update from downloading.

However, the grace period comes to an end on April 12. Microsoft has posted a warning on its Web site, alerting people about the impending deadline.

“Time is running out!” Microsoft said. “Please note that the mechanism to temporarily disable delivery of Windows XP SP2 is only available for a period of 240 days (eight months) from Aug. 16, 2004. At the end of this period (after April 12, 2005), Windows XP SP2 will be delivered to all Windows XP and Windows XP Service Pack 1 systems.”

Microsoft first released the SP2-blocking tool in August, saying it would allow customers to put off the upgrade for 120 days. In September, Microsoft said it would double the length of time, to 240 days, to give businesses more time to test the software.

In a statement, Microsoft said it is sticking to that schedule.

“SP2 is an important, free security update for Windows XP customers that is already delivering value to over 180 million customers worldwide,” said Jon Murchinson, a Microsoft product manager.

The move affects only those who use Windows’ automatic update to connect directly to Microsoft servers. Some businesses have reconfigured the automatic update feature to connect to their own corporate update servers. Those companies can continue to push out updates to their own schedule.

While recognizing the need for customers to test software, Microsoft has been urging businesses to move to SP2 for its enhanced security benefits, particularly for portable machines that frequently travel in and out of a corporate firewall.