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If you can log in or are logged in

(click on link below) Download and install  the Login & Keychain Update 1.0 for Mac OS X 10.5 Leopard to avoid the issue later.

If you are unable to log in

you can use these steps instead:

• Restart in Single User mode (hold Command-S during startup). Note that single-user mode always uses the US English keyboard layout.
• At the prompt, type this followed by Return: mount -uw /
• Type this:

• launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist

• Press Return
• Type this, then press Return: ls /Users
• Look at the listing and note the short name (username) of the affected user account.
• Type this:
  
  dscl . -delete /Users/username AuthenticationAuthority
  
  Note: Replace "username" with the affected user account's short name you noted above.
• Press Return
• Type this, followed by Return: passwd username
  
  Note: Replace "username" with the affected user account's short name you noted above.
• At the "New password:" prompt, type the user's password, then Return. Note: It is recommended that the original user's password be used to match the keychain password.
• At the "Retype new password" prompt, type the same password, followed by Return.
• Type this, followed by Return: reboot