One rather nasty consequence of running a screensaver as your desktop picture (see
this hint) is that you can no longer use the screensaver to lock out your system. There's a relatively simple way to get around this. Navigate in the Finder to /System -> Library -> Frameworks -> ScreenSaver.framework -> Versions -> A -> Resources, and then duplicate
ScreenSaverEngine.app. Move the copy wherever you want (I keep mine in /Applications/Utilites). If you'd like, you can rename it as well. Put it in your Dock or assign it a keyboard shortcut with Youpi Key. When you want to lock your screen, just activate your duplicated engine.
Depending on how you set up the screensaverengine to put the screen saver on your desktop, you should be able to configure the "real" screensaver using the Screen Effects prefPane. Otherwise, the lockout screensaver will be the same one that you use as your desktop background.