If you remove a processor from a multiprocessor computer or turn off hyperthreading support in the BIOS, and then try to change your Hardware Abstraction Layer (HAL), you only have the following options:
If the MPS Multiprocessor HAL is currently installed you can change to the following options:
If the ACPI Multiprocessor HAL is currently installed you can change to the following options:
Advanced Configuration and Power Interface (ACPI) PC
You do not have the option to switch to MPS Uniprocessor or ACPI Uniprocessor.
This behavior occurs because Windows Server 2003 and Windows XP only permit you to change to reliable HAL types.
To switch to a uniprocessor HAL, reinstall the operating system.
In Windows 2000, you can change to any listed HAL type. However, if you select an incorrect HAL, the computer may not start correctly. Therefore, only compatible HALs are listed in Windows Server 2003 and Windows XP. If you run a multiprocessor HAL with only a single processor installed, the computer typically works as expected, and there is little or no affect on performance.