Windows XP 32/64 bit any version only
After you change the number of processors in Virtualbox, you may not be able to use all the processors assigned to the Guest OS (Windows XP).
In task manager, there is only "one CPU", but there is two cores in the device manager.
Use this HALu.zip, which can be downloaded from:
Start the program, select "ACPI Multiprocesor PC" and restarted Windows. After reboot I can see two CPUs in Windows.
After the fix and a restarts, two cores are recognized in the Task manager of the guest OS.