Unauthorized password change

Has anyone had a problem with an unauthorized password change in the password
manager for Vista 32bit? It states that my password was recently changed and
i cannot log onto it now. I couldn't remember changing it, but just to be
safe i tried every password i use and none of them are correct. Could this be
an attack on my computer or something simple i am overlooking? Also I'm
apparently going to need to change it now, so does anyone have a link or
explanation on how to do that without knowing the current password?