Why would a current system with an Athlon 3200+ be installed without ACPI?

Well there are Motherboards where in bios default option for ACPI is "disabled" . So somebody can easilly overlook this and install windows without ACPI support. There is also proof that non-ACPI systems can gain in overall performance. So there are single core users who prefer non-ACPI. Also there are people who upgraded from Windows 98...