HP Thursday confirmed that some users of its AMD-based desktops have had problems after installing Windows XP Service Pack 3 (SP3), and said it would issue a patch this week to prevent machines from spiraling into endless reboots. HP also told users to delay installing XP SP3 until that patch was released.

Microsoft, meanwhile, acknowledged Thursday that it's working on a hotfix of its own.

The confirmations were the latest additions to the weeklong saga of problems some users have encountered after upgrading Windows XP to SP3. Last week, reports began showing up on Microsoft's support forum of "endless reboots" crippling machines running Advanced Micro Devices (AMD) processors. Many of the users said that the out-of-control PCs were from HP.

Users, led by Jesper Johansson, a former program manager for security policy at Microsoft and currently an MVP (Microsoft Most Valuable Professional) who works at Amazon.com, identified several causes, including one limited to HP-branded systems. According to Johansson -- and later, Microsoft itself -- HP used a disk image created on an Intel-powered machine to factory-install Windows XP on AMD-based PCs. Microsoft had advised computer makers against doing that as long ago as 2004.

An errant reference in Windows Registry for an unnecessary device driver -- "intelppm.sys," a power-management driver designed only for Intel-based PCs -- causes the XP SP3 upgrade to install that driver to AMD systems, said Johansson. That causes the PC to fail to reboot when it restarts after the update. Because most XP machines are set by default to reboot on a failure, the PC reboots repeatedly; some users have had trouble interrupting the endless reboots and regaining control of their computers.

HP did not explicitly admit the problem was its fault, but confirmed some details of Johansson's analysis. "The affected HP systems do not have an Intel driver loaded onto them, but there is a services registry entry that SP3 appears to be recognizing as an instruction to load the Intel driver, subsequently causing the failure," HP said in an e-mailed statement Thursday.

"HP is working diligently with Microsoft on a software update and will be proactively distributing a patch this week through HP Update that will prevent this error from occurring," the company continued. "HP recommends consumers with AMD-based desktops wait until after HP's or Microsoft's updates have been deployed on their systems to install Service Pack 3." The patch will be posted to this page of HP's support site when it's available.

"Microsoft is also developing a prerequisite fix that must be downloaded before SP3 will automatically install prior to its proactive distribution of SP3," HP statement added.

The Microsoft update that HP referenced is in the works, a Microsoft spokeswoman confirmed Thursday. "Microsoft is developing a hotfix for this issue, and will be available after it has been rigorously tested and meets our quality bar for release," she said in an e-mail Thursday afternoon.

Neither HP or Microsoft provided any details on what the Microsoft hotfix would do, but the "prerequisite fix" phrasing likely indicates the patch would be applied to either selective PCs or all XP machines before they are allowed to receive SP3 in the coming weeks when Microsoft flips the switch for automatic downloading and installing via Windows Update.

Microsoft has had to release several similar prerequisite updates or filters this year to prevent some users from obtaining service packs through Windows Update (WU). Last month, for instance, it delayed XP SP3 from reaching WU until it could craft a filter to exclude machines running its retail point-of-sale software. Microsoft also blocked significant numbers of users from receiving Windows Vista SP1 from WU beginning in late March.

Users impatient with HP's or Microsoft's patch plans can instead download a free tool crafted by Johansson that detects and fixed PCs that may be susceptible to the endless reboot issue.

Computerworld is an InfoWorld affiliate.