How to add support of multi-core/multiprocessor support to Windows XP SP3?

Run this in command line:

rundll32 syssetup,SetupInfObjectInstallAction ACPIAPIC_MP_HAL 128 %windir%\inf\hal.inf

This command replace single-core kernel to multi-core kernel without reinstalling Windows. You should be required to confirm driver installation and to reboot twice.

Check boot.ini to ensure that /NUMPROC option is not set to use maximum of cores or set it to required value. The msconfig tool provide GUI to change this option also.

This worked for me in VirtualBox 4.3 on Ubuntu.