Search This Blog

Monday, November 19, 2012

VirtualBox - upgrading Windows XP VMs to 2 CPU

Symptom: If you had Windows XP VM with only 1 CPU created / virtalized earlier you won't be able to see a second CPU when you add it later.

1. Make sure you have second CPU added via VM Virtual Box Manager and "Enable IO APIC" checkbox  in System settings on Motherboard tab is checked.
2. You should see 2 CPUs in Device Manager , but when you run Task Manager, you'll see only 1 CPU.
3. MS recommendation for Windows XP is to reinstall Windows (no surprise) as you can see it in this KB article:
4. Do not reinstall Windows, just run run the following from command prompt

rundll32 syssetup,SetupInfObjectInstallAction ACPIAPIC_MP_HAL 128 %windir%\inf\hal.inf
5. Reboot when prompted , log on back to Windows XP wait until you see "Windows has finished installing new devices. The software that supports your device requires that you restart your computer. You must restart your computer before the new setting will take effect" and press OK to reboot it again.
6. Done


  1. Thanks Alex. It worked perfectly here.

  2. Really GREAT !!
    2 minuts instead of a full reinstallation or a slow old XP !!

  3. Perfect, just what I was looking for, many thanks!

  4. Worked perfectly, thank you for sharing this solution.

  5. Hello, to reverse this, is it as easy as just selecting 1 processor under VirtualBox's processor settings, or do we have to run some kind of 'reversal' command on the guest PC's? Thanks

  6. Brilliant, thanks, only problem was took me few minutes to realise text is case dependent! Beats me how you came up with this.

  7. useful! thanks for sharing! :)