If you log in each time the computer starts up, do the following:

	1.	Start up the computer in Single-User Mode.
	2.	Check the volume with fsck.
	3.	Press Return.
	4.	Type: mount -uw /
	5.	Press Return.
	6.	Type: cd /Users/[affected username]/Library

Tip: Instead of "[affected username]", type the username for your account, or the account that is experiencing this issue. For example, if the username is "theta," type: 
cd /Users/theta/Library
	7.	Type: mv Preferences Preferences_old
	8.	Type: reboot
	9.	Press Return.

If you can successfully login, use the following steps to isolate the issue:
	10.	Open the Console utility. It's in the Utilities folder (/Applications/Utilities).
	11.	From the File menu, choose Open Log.
	12.	In the goto field, type: /var/log/system.log , then click the Open button.
	13.	From the Edit menu, choose Find.
	14.	In the Find window, type "parse failed" and click the Next button.
	15.	Look for log entries that are similar to the following:
	16.	For each "parse failed" message that you find, remove the matching .plist file from the /Library/Preferences_old folder.
	17.	Repeat steps o and p until you have removed all of the .plist files associated with "parse failed" messages.
	18.	In the Finder, rename /Library/Preferences folder to Preferences_new.
	19.	Rename Preferences_old to Preferences.
	20.	From the Apple menu, choose Log Out. When the login panel appears log back in.