If unchecked, windows will use the maximum number of cores that it can during the boot processes. You only need to change this setting if you specifically know it is causing problems. It's one of those crazy internet misconceptions that microsoft have got this wrong - you will even see posts by people swearing that their boot is faster when playing with this setting. leave it as it is if you do not have any boot problems. http://www.eightforums.com/drivers-hardware/5113-help-advanced-boot-options-number-processors.html