"When you are installing Windows 2000, you may receive the following error message:
STOP: 0x000000A5 (Parameter1, Parameter2, Parameter3, Parameter4)
The ACPI Bios in this system is not fully compliant to the specification. Please read the readme.txt for possible workarounds, or contact your system vendor for an updated bios.
The bios in this system is not fully ACPI compliant. Please contact your system vendor for an updated bios. If you are unable to obtain an updated bios or the latest bios supplied by your vendor is not ACPI compliant, you can turn off ACPI mode during text mode setup. To do this, simply press the F7 key when you are prompted to install storage drivers. The system will not notify you that the F7 key was pressed - it will silently disable ACPI and allow you to continue your installation. Note that Parameter1, Parameter2, Parameter3, and Parameter4 vary from computer to computer."
See Knowledge Base article Q256841 for more details.