In my work (and play) I use a lot of VMs for various reasons. I recently installed a minimal Windows XP Professional VM and forgot to give it access to more than one CPU core. This causes the Windows kernel to be compiled with no Hardware Abstraction Layer (HAL) for multi-core CPUs. I shut off the VM, added another CPU core, and it refused to use it. I thought I only had two choices at this point. I could stick with one core, or do a new install. With a little research I found that there is actually a program to patch the kernel to add HAL support!

The original article (and files) are here: https://forums.virtualbox.org/viewtopic.php?f=7&p=111649.  Just in case this thread ever disappears, I’ll post the instructions and file here.

xWarning! Windows XP Professional is the only version of XP which supports multi-core processors. If this fails, you may have to re-install Windows.
  1. Download the HAL Updater – HALu-0.2.0.zip
  2. Extract the files to a directory where there are no spaces in the path (such as C:\HALu-0.2.0\)
  3. Run HALu.exe
  4. Select the appropriate HAL (for dual-core it is almost always “ACPI Multiprocessor PC”)
  5. Click “Update”, and your current HAL will be replaced by the one selected
  6. Reboot