Ubuntu :: Log In Failing On Most Users

recently logging in become a bit unreliable - i thought i was mis-typing the passwords but on closer looking the passwords were ok (faulty passwords produces "authentication failure) Using the correct passwords the screen blanks for a second and then returns to the list of possible logins. I cannot log in except as one of the children - could this be because their logins have been used less often and less recently Everything was working well and I have not knowingly changed anything except the passwords after the problems started in order to solve what i thought was a simple mis-typing problem

I can log in as the children but their logins obviously do not have permission to do much
I also have a bootable puppy CD Unfortunately I am also a total noob but determined not to backslide into XP because up to now Ubuntu has been fantastic