I agree with alibi and ishaan. Actually while hibernating ur PC, the system stores whatever data is there in RAM to a file hiberfil.sys, that's why size of this file is exactly equal to the RAM in ur PC. During shutdown, the OS "SAVES" everything, i mean the settings and changes u made in the system. So once a while u shud use shutdown also. A word of caution(my experience)....once i hibernated my system and took my HDD to a friend and copied a couple of movies. When i switched back, i cudn't find those movies. It occurred bcoz system tries to get into original(previous) state after hibernation. Please correct me if I am wrong on this. Otherwise, hibernation is cool option...