This isn't my mac but my mums, recently everytime she logged into her account it would load the desktop then straight back to the login screen. She could only login to her guest accounts!. I couldn't fix her admin login, so i created her a new admin account which was a great deal of faffing about with command prompts and folder rights. I basically had to tell the computer it was a new system, i then copied everything over to the new account. Now a week later after everything was working brilliantly, she's turned the system on and same again straight back to the login! I'm really starting to hate mac systems, they are a pig to fix and my mum is abit upset.