A client's new PowerMac G4 running 10.2.8 decided it wasn't going to shut down or reboot or logout. Selecting any of those items from a menu, or pressing Control-Eject, produced no response.
The user's loginwindow.plist file (in the user's Library -> Preferences folder) had become corrupted and was full of garbage. Removing the file cured the problem. A sudo reboot was required for the first restart after removing the file.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031023115102913