wasn't sure whether to put this in hardware or software....
The recent Windows XP Service Pack 3 update has left certain AMD-based PCs manufactured by Hewlett-Packard caught in an endless reboot cycle caused by an Intel-specific disk image mistakenly being used with AMD hardware. According to the Microsoft Update Product Team Blog, "The problem is a registry value, present on images created with Intel processors, that causes a driver (intelppm.sys) to load at boot."
In order to fix the problem, Microsoft has temporarily blocked these AMD machines from getting the service pack while it cobbles together an alternative update. While some users wait for Microsoft's fix to eventually be released, others have turned to a former Microsoft employee, Jesper Johansson, who coded up a free tool to detect and fix the problem and made it available last Wednesday. Once Johansson's fix has been implemented, users can update their systems to SP3 without any problems.
So while Microsoft is busy "investigating a fix" that's basically been readily available from one of its former employees since the middle of last week, consumers can also update their HP computers with a patch that was deployed last Wednesday, available here. To be fair, Microsoft does outline steps to resolve the issue here, but the process involves manually starting your computer in Safe Mode and tinkering with your registry, two tasks that are far beyond the skill set of most average computer users.