Reports are all over about problems with some users installing Windows XP SP3; their systems go into a continuous reboot cycle.
Now Microsoft has provided an explanation, and it’s an interesting one. The problem occurs on AMD-based systems where Windows was built from an OEM image captured on an Intel-based system. Perhaps it never mattered before, but in such cases, and depending on a particular registry key, an Intel-specific driver (Intelppm.sys) is loaded.
Microsoft Knowledge Base article KB88372 explains this some more and gives instructions for what to do if you are experiencing the problem.