I checked your posting history and I assume this is the powermac G4 you got second hand, it came with Tiger and you installed Leopard on a second disk.
When you installed Leopard did you have it transfer the user settings from the other HD?
Because that seems to be the problem, you're still using the settings from whoever had your computer before and you're stuck with them as your administrator.
When you get a second hand computer it's best to clean it off entirely, get rid of the last users accounts and other crap.
It's probably best if you do a full reinstall of leopard without transferring any user settings, this will allow you to set yourself as administrator and start from a clean slate. Backup any documents, email, etc that you might want to keep beforehand.