I think Vista requires an ACPI compliant system so you're probably screwed in that respect. Searching for "Vista 0x000000A5 site:microsoft.com" gives this:
If you receive this error message when you install Windows Vista, follow these steps:
1. Restart the computer.
2. Roll back the installation to the previous operating system.
3. Contact the computer manufacturer for information about how to obtain and to install a BIOS update.
4. Disconnect any external hardware devices that are connected to the computer. Additionally, remove or disconnect any hardware that the computer manufacturer does not support.
5. Try to upgrade to Windows Vista again. If you receive a "Stop: 0x0000007B" error message during the upgrade process, contact the computer manufacturer. If you can upgrade to Windows Vista, the problem was likely related to hardware or to software that was installed in the previous operating system.