There are people who don't have HPET just ACPI APIC. If one disables that (in the bios) using XP you will bork your install of XP. With Win7 I would assume the same thing although I don't recommend anyone trying that. The only way I know to change ACPI APIC is a complete OS reinstall. But I don't recall an option to change HAL when doing that either though.