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 summarised 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 has 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.

However, HP has posted a document to its support site that shifts the blame right back to Microsoft. "After installing the initial release of Service Pack 3 for Windows XP an error condition can occur," the HP document reads. "The Service Pack 3 update copies an Intel power management driver to the computer that was not on the computer before the update [emphasis added]. During Windows startup, computers with AMD processors may experience a blue screen error."

Computerworld has not been able to confirm that the errant driver is, as HP claims, added by XP SP3 to AMD-based PCs. If that is, in fact, the case, the endless rebooting is Microsoft's fault, not HP's.

Is the intelppm.sys driver problem the only AMD-specific reboot issue? Not according to Johansson and other users, who have also reported shutdown error messages claiming that the PC's BIOS is "not fully ACPI compliant."

Users who have seen this message say that their PCs are running one specific motherboard - an ASUS A8N32-SLI Deluxe - equipped with an AMD processor. Those same users claim that inserting a USB flash drive or connecting another USB-based storage device before booting solves the problem.

Are some Intel-powered PCs experiencing endless reboots?

Yes, say users.

Johansson noted that some users had seen error messages that indicate their crashes are video driver-related. Both ATI and nVidia drivers may be involved, Johansson said.

How can I stop the endless reboots?

To break the chain, you may first need to halt the automatic restarts. Press the F8 key when you see the black Windows XP screen and then select "Disable automatic restart on system failure"

Now, try to get into Safe Mode by pressing F8 at the next reboot. You may need to press F8 repeatedly. Once in Safe Mode, follow the steps Microsoft lists in this support document to disable the intelppm.sys driver if you suspect that's the problem.

If that doesn't work, Johansson suggests trying the more advanced Recovery Console, which can be run from the XP install CD if it hasn't previously been added to the PC. For detailed directions on installing and using the Recovery Console, check out this Microsoft support document . Again, disable the intelppm.sys driver.

If your PC is not an HP or Compaq with an AMD processor, your best bet - in lieu of anything official from Microsoft - is to read through Johansson's post for possible solutions.

Isn't there an easier way?

We're not sure, but you can try to offload the heavy lifting to Microsoft by using the free tech support it's offering for any install or upgrade problems related to XP SP3.

Although Microsoft normally refers users who obtained XP as part of a new PC to the computer manufacturer or reseller when problems pop up, it will provide free-of-charge XP SP3 install support until 14 April, 2009. You can contact Microsoft by phone, email or online chat. (Email and chat are available only in the US and Canada.)

Details can be found here.

What if I just want to give up on XP SP3 and revert to something that works?

No problem, as long as you can regain control of the PC.

Assuming you can stop the endless reboot spiral, you can ditch SP3 and return to (presumably) SP2 by opening "Add or Remove Programs" from Control Panel, checking the "Show Updates" box, then scrolling to the bottom of the listing. Select "Windows XP Service Pack 3" and click the Remove button.

The PC will reboot (again!), but after that, the machine should return to its pre-SP3 state.

Microsoft spells that out for you in this recently-updated support document , which also walks you through winding back to SP2 using System Restore or the Recovery Console.