I've disabled ACPI in my BIOS now Windows 2000 will not boot.

A. During installation if Advanced Configuration and PowerInterface (ACPI) is detected specific changes are made to the configuration inthe form of registry entries and system files.

If you then disable ACPI in the BIOS the OS will fail to boot with a BSOD:

0x00000079 (0x00000004, 0x0000AC31, 0x00000000, 0x00000000)

This is because a separate HAL is used for ACPI, non-ACPI systems. If you nolonger want ACPI you will need to reinstall Windows 2000.