In the last month or two a bug seems to have appeared in the login software of our fully up-to-date Windows XP Pro installation. We have the login screen set to display the users' icons. If I log in as an administrator and then l log out again and try to log in as another user, then the login screen does not detect the keypresses used to enter passwords. It might be that logging in as any user and then logging out again is enough to produce the problem, I haven't tried it (and I don't care enough to do so).
There is a work-around: pressing Ctrl-Alt-Delete twice brings up the old Windows NT login box and this does detect keypresses correctly.