Virtual Box - Windows XP won't see more than one processor

[Mark]

If you installed XP with only one processor, and then raised the processor count afterward XP won't recognise the new processors. It has to do with the HAL chosen by XP when it's installed. There's single-processor and multi-processor versions.

Using devcon.exe ("the command line version of the device manager") Forced HAL Upgrade

1) Download devcon.exe utility from Microsoft
http://support.microsoft.com/?scid=kb%3B...&x=13&y=11

2) Extract the downloaded devcon.exe (I used 7-zip)

3) Open a command prompt and cd to the directory where you extracted devcon.exe and find appropriate folder (x86 / x64) (Mine was: C:\temp\devcon\i386) and run these commands.

devcon.exe sethwid @ROOT\ACPI_HAL\0000 := +acpiapic_mp !acpiapic_up

Output:
ROOT\ACPI_HAL\0000 L acpiapic_mp,acpipic_up
Modified 1 hardware ID(s).

devcon.exe update c:\windows\inf\hal.inf acpiapic_mp

Output:
Updating drivers for acpiapic_mp from c:\windows\inf\hal.inf
Drivers updated successfully.

After a reboot, XP re-detects all hardware, and then requests another reboot. XP can see all 4 cores.


Additional

One of the recommended "fixes" was to re-install..

Some light reading: HAL options after Windows XP or Windows Server 2003 Setup
http://support.microsoft.com/kb/309283

After reboot
Popup from taskbar tray.
Found New Hardware: ACPI Multiprocessor PC