If you use a different keyboard layout to the standard QWERTY keyboard you may still be trying to type in your password in the QWERTY layout before Windows loads your personal settings and you are able to use the layout you like.
This is because Windows stores the keyboard layout setting per user account. To change the default keyboard layout that is used at the logon screen (eg, when switching to a Dvorak keyboard layout) edit the Regestry key
HKEY_USERS\.Default\Keyboard Layout\Substitutes (and maybe Preload)
using the data from the
HKEY_CURRENT_USER\Keyboard Layout\Substitutes key.