Prescott and XP SP2 Update
It seems there are plenty of people out there running Prescott processors without the latest BIOS code. Seems that a number of vendors were putting out boards with pre production microcode versions. And XP needs to see version 8 or later
So go get the latest BIOS update. Unless your mobo manufacturer hasn't yet released one (which is the case for a couple of them) In that case a workaround is available here