To enable ACPI after Windows 98 is installed, run Regedit and go to the following key in the registry:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Detect
Create a new DWORD value called "ACPIOption" and set it equal to 1.
In Control Panel, run Add New Hardware and choose hardware detection. ACPI should be detected and installed. After the system is rebooted, all ACPI-enumerated devices will be set up again.