I have to let everyone know about this
Having upgraded a few of the computers at work (2 of us are network admin and I run a computer lab) I/we had nothing but problems. I did a little research on reported problems and issues with the upgrade......
Microsoft faults OEMs for some XP SP3 endless reboots
Microsoft document claims the fault for endless reboots lies in the Windows XP image originally installed on the PC by the computer manufacturer
Microsoft blames computer makers for some of the problems users have encountered after updating to Windows XP Service Pack 3 (SP3), according to a company support document.
The document also showed that the "endless reboot" problem some users have reported after installing XP SP3 was neither unanticipated or new; Microsoft updated the document the same day it released the service pack, and indicated that the same thing happened nearly four years ago when it rolled out Windows XP SP2.
Knowledge Base document 888372, last updated May 6, spelled out an error message that stops a PC's boot process -- and, depending on the machine's settings, may make it repeatedly reboot -- after installing SP3. The fault, said the Microsoft document, is in the Windows XP image originally installed on the PC by the computer manufacturer, or OEM.
MORE.....
FAQ: What you should know before installing Windows XP SP3
Good luck and have fun....
EDIT: It may not just be limited to HP's and/or AMD processors.