Users running Windows 7 or Vista might experience a Black Screen when they attempt to unlock their machine in certain conditions, Microsoft warned. "» Vista or Win7 running "Aero" graphics mode. » Vista or Win7 PC is subject to additional security restrictions and security software, such as might be the case in enterprise, or with mandated security settings and software such as DISA or DoD requirements. » PC is locked and left undisturbed long enough for screen to go black, which's 20 minutes with default power policy of "Balanced". » A person unlocks PC by entering their user credentials -- the screen goes black, and remains black for as much as 10 minutes or longer. » The machine isn't "hard-locked", as periodic disk-drive activity may be noted from the disk-drive indicator light on the computer, if so equipped," explained Microsoft in KB2431700.