Installing Windows XP Service Pack 3 sends some PCs into an endless series of reboots, according to posts to a Microsoft support forum.

Jesper Johansson, a former program manager for security policy at Microsoft and a prominent Windows blogger, has worked with users to tentatively identify the problem as involving only machines using processors from Advanced Micro Devices.

Messages from frustrated users began accumulating on the XP SP3 support newsgroup Wednesday, just a day after Microsoft released the update to the general public.

"I just installed Windows XP SP3 and after completing the processes and when the system reboots, the system cannot proceed to load the Windows," said a user labeled as "Olin" in a message that kicked off a long thread. "It just displays the flash screen of Windows then after it reboots again."

Most users who left messages on the forum said that they were unable to boot into Windows Safe mode -- a last-ditch way to sidestep the normal boot process for troubleshooting purposes -- or revert to a previously saved System Restore point.

Some were understandably upset. "Way to go, Microsoft, releasing the pile of dung called SP3 that hoses your system so bad even Safe Mode isn't working!" said a user identified as Mike Voss. "Props to your QA guys, they certainly have done their job."

Johansson, who watched one of his PCs repeatedly reboot after installing XP SP3, traded accounts with several other users on the newsgroup, and summarized the results on his blog.

According to Johansson, there appear to be two separate issues. One affects only AMD-equipped PCs sold by Hewlett-Packard. "The problem is that HP, apparently along with other OEMs, deploys the same image to Intel-based computers that they do to AMD-based computers," said Johansson. "Because the image for both Intel and AMD is the same all have the intelppm.sys driver installed and running. That driver provides power management on Intel-based computers. On an AMD-based computer, amdk8.sys provides the same functionality."

Running the "intelppm.sys" driver on an AMD-powered PC isn't normally an issue, but on the first reboot after a service pack installation, it causes "a big problem," Johansson said. The machine either fails to boot or crashes and immediately reboots.

The other problem, according to Johansson, also seems to affect only AMD machines, and involves an error message indicating trouble with the PC's BIOS. Johansson said that the ensuing recommendation to update the BIOS is "most likely not your problem," but said that the problem may be isolated to a specific motherboard. "Possibly, it is related to computers with the ASUS A8N32-SLI Deluxe motherboard in them," he said.

Johansson also spelled out workarounds for both problems on his blog. The HP issue can be solved by disabling the intelppm.sys driver, while the second fix requires the user to plug in a USB flash drive before booting.

Microsoft was not immediately available for comment early Friday, but someone identified as a Microsoft employee on the support forum had asked users to e-mail him information such as the PC's system configuration, tell him whether they were able to enter Safe mode, and submit event viewer logs.

This isn't the first endless reboot problem Microsoft's faced in relation to a service pack recently. In February, the company pulled a Windows Vista SP1 prerequisite update from automatic delivery because it was crippling some machines.