Windows 8 doesn't have the F8 function enabled by default - to get into safe mode, you must first get into windows. To enable the F8 function you have to run a command to enable the feature, then F8 will work.
just to check, numlock isn't switched on is it, causing a problem? If its a laptop without a number pad, half the keyboard will be a number locked if you press the numlock key, meaning when you type letters on one side, they are numbers - which you wont see if you are typing a password. Check this, to turn off num lock press Fn NumLock, probably highlighted blue on the key print.
if you haven't already, try enforce an improper shutdown, this will make windows enter a repair mode which MIGHT help. To do that, turn off laptop, unplug battery, plug in the charger and switch on. As the laptop is booting, pull the plug. Now plug back in and switch on....