WInXP SP2 problem with Prescott processors !

What will happen if I install SP2 on an affected PC?

The installation of SP2 will go fine, until it prompts to restart. Then every attempt to boot XP (Last Known Good, Safe Mode Command Only, the works) will lock up hard before you ever see the desktop. Fortunately, this happens before file system writes are pending, so the file system doesn't seem to get corrupted.

If you are doing a "normal" XP boot, you will see the black GUI splash screen with the progress indicator bar that normally moves back and forth. In my case, this always locks up about 2/3 of its first pass from left to right.

What is the problem?
Normally, Windows XP will run whether the BIOS updates Prescott's microcode, or just leaves it at Revision 0. But the new Update.sys installed as part of XP Service Pack 2 will hard lockup the PC if Prescott's microcode is left below Revision 8 (or for some Prescott steppings, Revision 7).