According to http://technet.microsoft.com/en-us/library/cc782277(v=ws.10).aspx this should be simple. Trying it on a Server 2003 VM (that had been converted from Virtual Server 2005) didn't give the options of the other HALs. The answer is at http://www.pimp-my-rig.com/2008/08/article-acpi-uniprocessor-to.html - use the command line devcon.exe tool and the .cmd script at http://www.pimp-my-rig.com/2008/10/acpi-multiprocessor-hal-upgrade-script.html .
Devcon can be downloaded from Microsoft at http://support.microsoft.com/kb/311272
The script is:
@echo off @title "Upgrading to ACPI Multi-Processor HAL.." cls echo ==================================================== echo Upgrading to ACPI Multi-Processor HAL.. echo ==================================================== echo. echo please wait.. devcon sethwid @ROOT\PCI_HAL\0000 := !E_ISA_UP !ACPIPIC_UP !ACPIAPIC_UP !ACPIAPIC_MP !MPS_UP !MPS_MP !SGI_MPS_MP !SYSPRO_MP !SGI_MPS_MP > nul devcon sethwid @ROOT\ACPI_HAL\0000 := !E_ISA_UP !ACPIPIC_UP !ACPIAPIC_UP !ACPIAPIC_MP !MPS_UP !MPS_MP !SGI_MPS_MP !SYSPRO_MP !SGI_MPS_MP > nul devcon sethwid @ROOT\PCI_HAL\0000 := +ACPIAPIC_MP > nul devcon sethwid @ROOT\ACPI_HAL\0000 := +ACPIAPIC_MP > nul devcon update %windir%\inf\hal.inf ACPIAPIC_MP > nul echo. echo ==================================================== echo Script Completed: press any key to reboot.. echo ==================================================== pause > nul devcon reboot