your USB keyboard may not work because your laptop has not got to the point of initiating USB at the point of booting to password.
You could try using ascii codes to eneter the key
at the point where you need to enter the key that doesnt work press your numlock button
this will turn some of your letters into numbers...they should be written on the keys in a different colur...the laptop Im looking at at the moment has 4 on U, 5 on I 6 on O, 1 on J etc...
hopefully these are not the broken key..
if you look here you will see the chart that says
find the letter you need eg v=118 in lower case
type in your password till you get to the v (if thats the broken letter)
press and hold [ALT] button with num lock on and on your newley formed number pad type in the number thet represent the letter in this case 118 when you release the [ALT] key it will insert that letter.
however this only works with an extended keypad (like a full size keyboard) where the numbers are seperate not your top row numbers which is why you need your numlock on.
if it does then work maybe you could go into your bios and turn USB legacy support on...which may then help your full size keyboard to work too.