When I try to log in under a particular user account, the login window shudders and I can't log in.
The most likely cause of this error is that you are entering the user account information incorrectly. If you are selecting a user account from the list, you must be entering the password incorrectly. If you are entering both the account name and password, you have to ensure that you use the correct combination. Try entering the information again. Remember that both name and password are case sensitive.
If you are logging in using the short name, try using the full name instead.
If you are still not able to log in under that account, log in as the administrator and open the Users pane of the System Preferences utility to check the account or edit it. You can also delete the account and start over.
If you can't log in to an administrator account because you have forgotten the password, do the following steps:
Restart the Mac using the Mac OS X Install CD 1.
When the installer opens, select Installer, Reset Password.
Follow the onscreen instructions to reset the administrator password.
When I open the Accounts pane of the System Preferences utility, the buttons are all inactive.
The Lock icon in the lower-left corner of the window enables you to lock or unlock the ability to make global system changes. If this icon is in the "locked" mode, you need to authenticate yourself as the administrator (even if you are logged in under that account). Click the Lock icon, enter the Administrator User Name and Password in the Authentication dialog box, and press Return. If you enter valid information, the buttons become active and you are able to make changes to the account configurations.