It needs to be turned off in your bios settings... you'll need to do a clean installation too. You might want to consider going to your m/b manufacturers site, looking for bios flash updates and seeing if the problem is addressed in any of the revision notes.
Just press F7 during the setup (or F5 to select proper HAL by hand) when you reinstall WinXP. No need to disable in the BIOS however. Windows will just install a Standard PC without the ACPI capabilities. It is possible to do this in a running Windows by selecting the Standard PC in Device Manager and deleting all remaining (hidden) devices. Don't know if it's wise, but no harm in trying I guess.