Add CPU to Windows XP
Yes, I realize XP is long gone, but I still use it in Virtualbox for some old apps that I have. I remember back in the day the word of advice to go from 1 CPU to multi was a reinstall as the kernel had to be swapped out to work correctly. Well, I found this article from a web search, and the following command did the trick. First disable hardware driver signing ( My Computer > Properties > Hardware...). Then run this at a CMD prompt: rundll32 syssetup,SetupInfObjectInstallAction ACPIAPIC_MP_HAL 128 %windir%\inf\hal.inf Hope this finds its way to help someone else.