XP Suddenly asking for password [duplicate]
Perhaps you added another user. This will cause it to ask for a password. It might be a hidden user.
Try using (WIN+R)
control userpasswords2
To see if anything unusual is there, and you can also make sure the "users must enter username and password to use this computer" box is unchecked.
I solved this using the Offline NT Password & Registry Editor. This tool is a lifesaver in situations like this.