System logs out automatically on login!
Since this is a common enough problem, submit feedback to http://www.apple.com/macosx/feedback for Apple to make the operating system startup notice when it takes too long to login, and to automatically delete the cache files which could cause this problem and retry to login.
If you have eliminated potential hardware conflicts, bad directories, bad ram, and kernel panics (links to the FAQ on kernel panics) from the possible cause for the system logging out.
you may need to follow one of these sets of instructions to regain control of your system:
1. Restart your machine holding command-S, where command key is the key with the Apple logo and cloverleaf icon.
2. When the prompt appears, type
followed by return key.
to make sure your directory is OK. If it reports errors, you'll need to repair the directory first.
3. type at the prompt
rm -i /Library/Caches/com.apple.LaunchServices.LocalCache.csstore
followed by return key, hit the letter y if it shows that file to be deleted.
4. type at the prompt
followed by return key
followed by return key
End of Instruction set 1
Instruction set 2 (only use if set 1 doesn't work):
1. Turn off the computer and Disconnect all network wires, peripherals, etc..
2. Boot off the Installer CD
3. Set up the password for root in the Reset password
4. Logged in as root.
5. duplicated Home folder, and removed all the preference files except bookmarks, favorites, and mail and internet settings from the copy of the home folder. The preferences are in home folder copy -> Library -> Preferences
6. Apple menu -> System Preferences -> Accounts deleted the user whose Home folder was corrupted.
7. Created a new user by the name of the old home folder.
8. Migrated the contents of the Home folder copy into the new home folder.
9. Logged out, and logged back in as the new user.
10. Removed the root account from the Apple menu -> System Preferences -> Accounts.
11. Repaired permissions with Applications -> Utilities -> Disk Utility -> First Aid tab -> Select hard disk, and hit Repair Permissions (only possible with 10.2 and up). With 10.1.5 you can do it with Repair Privileges Utility
Some applications may need their registration numbers reinserted, but your data should be safe.