I do that on a regular basis. I have an image of Windows XP that was built using a P4 2.4. Whenever I use that image on a newer PC, the HAL needs to be upgraded. An easy way to perform the test is to setup a VM with one CPU and upgrade to 2 CPUs once the installation is complete. You'll see that it changes from ACPI Uniprocessor PC to ACPI Multiprocessor PC. Don't do it on a production VM if you plan on going back to one CPU because the multiprocessor HAL has additional overhead that will cause minor performance degradation when using only one CPU.