Hi minsbang and welcome to Vista Forums
Have you checked that
Caps Lock isn't on (use
Shift to enter capital letters) and that
Num Lock is on if you use the numeric keypad to enter any numbers in your password. Make sure that the keyboard language setting is correct, as some symbols move position accordingly, and if your password has any of these then it could explain your problem - for example,
Shift 2 gives
" in UK English whereas in US English the same combination yields
@. And yes, I have been caught out with this in the past.