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Windows XP enable MultiCore after SingleCore installation

When you install Windows XP on a single core machine (virtual or physical) it remembers that CPU model. Even if you change the CPU or add more cores it will still just use a single core.

To make the system aware of a multicore CPU you can do the following trick:

1. Download the command line devicemanager utility (devcon.exe) from Microsoft (never version will work but make sure you use the correct version (x86/x64)).

2. Execute the following 2 commands:

devcon sethwid @ROOT\ACPI_HAL\0000 := +acpiapic_mp !acpiapic_up
devcon update c:\windows\inf\hal.inf acpiapic_mp

Then let the automatic hardware detection do the job and wait until you get prompted for a reboot. – Reboot

You should now see all cores!

Background information:
When changing the CPU/Cores Windows XP needs to update the HAL (Hardware Abstraction Layer). The above 2 commands do exactly this.

I needed this for a virtual Windows XP machine (VirtualBox), worked perfect!