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.