Which BIOS setting should be changed for a non-booting Windows 8.1 computer?

The recommended setting to change for a non-booting Windows 8.1 computer is to disable Secure Boot. This is because Secure Boot is a security feature that helps ensure that only trusted operating systems and software can be loaded during the system's boot process. If the operating system or its loader is not correctly signed or recognized as secure—possibly due to being custom, or an older OS like Windows 8.1—this can prevent the system from booting properly.

Disabling Secure Boot can allow the system to proceed with loading an operating system that may not meet the secure boot requirements, enabling troubleshooting steps or allowing a successful boot in cases where there are compatibility issues. Once you've resolved the booting issue, you could re-enable Secure Boot to maintain security during future operations.

Changing boot mode to Legacy may be useful in some cases, especially for older operating systems that were not designed to work with EFI-based systems. However, Windows 8.1 is compatible with UEFI, making this setting less relevant as a first action. Similarly, changing the HDD priority might help if the computer is attempting to boot from the wrong drive, but this does not address the potential underlying issue caused by Secure Boot.