If you remove processors from your multiprocessor computer, so that only one processor is left, you have a limited choice when you attempt to change your HAL (Hardware Abstraction Layer).

If the MPS Multiprocessor HAL is installed, you can switch to:

Standard PC
MPS Multiprocessor

If the ACPI Multiprocessor HAL is installed, you can switch to:

Advanced Configuration and Power Interface (ACPI) PC
ACPI Multiprocessor
MPS Multiprocessor

You cannot switch to the MPS Uniprocessor HAL, or the ACPI Uniprocessor HAL.

The only way to switch to these HALs is to reinstall the operating system.