Imac G4 800, Shutdown Doesn't Power Off


I'm running Mac OS X Version 10.3 on an iMac G4 800.

When I click on "Shutdown", (from both the login screen or when I'm already logged ) the screen eventually goes black, but I can still hear the fan running. I have to hold down the power on/off button in order to get the fan to power off.

Any clues as to how to get my system to shut down "normally"?