If you used system cache cleaning software it can mess up Apple software. Stop using such software. Remove Mackeeper if present using these directions:
If you started typing something in the search field of the System Preferences, it might be hiding. Quit and reopen System Preferences to be sure it isn't hiding because it is searching for another panel.
Lastly, if none of the above apply, go to your hard drive -> System -> Library -> PreferencesPanes folder and see if it is hiding there. If it isn't, get your 10.5 installer disc that installed 10.5 on your Mac, and restore it with http://www.charlessoft.com/ 's Pacifist, and then run the 10.5.8 combo update to ensure the version matches the operating system.