Microsoft's having a tough year with reboots. First it was a reboot ad infinitum in February, brought on by a flawed update to Windows Vista . Now the same thing's happening to some users who have updated to Windows XP Service Pack 3 (SP3).

What gives?

We don't have the answer to that -- at the moment, no one seems to have the definitive answer to that. But we do have answers to the most pressing questions about the latest Windows snafu, including a developing game of blame.

What's happening? After some users update to Windows XP SP3, their PCs reboot -- which is normal -- but then fail to start up, and then reboot again and again, which is decidedly not normal.

The "endless reboot" cropped up immediately after Microsoft made XP SP3 available to the general Windows-using public by posting the service pack to Windows Update last week. Within a day, users were reporting problems in messages on Microsoft's XP SP3 support forum.

Why are some PCs rebooting endlessly? Microsoft hasn't given any official explanation, but users on the support forum sharing accounts have identified several possible causes. Some seem to affect only systems running processors made by Advanced Micro Devices (AMD), while others have also hit users with Intel-based PCs.

Although it's possible to get a feel for the theories by reading multiple support forum message threads, the best source of information has been a frequently updated blog post by Jesper Johansson, a former program manager for security policy at Microsoft and currently an MVP (Microsoft Most Valuable Professional). Johansson, who had one of his own PCs slip into a reboot coma, has neatly summarized the several possible causes put forward by himself and other users.

Are only AMD-powered machines sold by HP rebooting over and over? No, although they've gotten the most press.

According to Johansson and others, Hewlett-Packard used the same Windows XP disk image to factory install the OS on AMD-based systems as they used for PCs running Intel processors. That's a mistake, Microsoft contends.

"Under this configuration, after the computer is upgraded to Windows XP SP2 or SP3, the Intel processor driver (intelppm.sys) may try to load because an orphaned registry key remains," Microsoft said in a support document first released in 2004, after the company issued XP SP2.

The presence of the unnecessary driver, said Microsoft, may crash the machine, causing it to reboot. If the PC is set to automatically reboot on a start failure -- as most are by default -- it reboots endlessly, often so quickly that the user can't interrupt the process and enter what's called "Safe Mode" in Windows, a last-ditch way to sidestep the normal boot process for troubleshooting purposes.

So Microsoft's blaming HP? It sure sounds like it. Yesterday, a Microsoft spokeswoman confirmed that the company was aware of the reboot problem, and pinned responsibility on the computer maker, but didn't name names. "Microsoft issued guidance to OEMs [computer manufacturers] advising them to only load Windows XP images onto like hardware in 2004," she said.

What's HP said? Nothing and a lot, if that makes sense.

Although Computerworld asked for comment on Monday and an HP spokeswoman said Tuesday that the company was investigating, no one from HP had followed up as of end-of-day on Wednesday.