But as a former Linux geek I'm used to things not working and things that need tweaking, so I created another user, logged in with that user and changed the screen saver settings. Then with Sherlock I searched for all files changed today. I found an interesting file in Library/Preferences/ByHost called com.apple.screensaver.*.plist. It's an XML file containing the screen saver settings.
Then I logged in again with my regular user, opened the Finder and opened the same file for my regular user. By double clicking, the 'Propery ListEditor' launched, with which I could change the properties in this file. There are two properties, moduleName and modulePath. For moduleName I filled in 'Beach' and for modulePath I filled in '/System/Library/Screen Savers/Beach.slideSaver' (which I copied from the other user).
Now when I open the Preferences app and look at the screen saver, it doesn't crash anymore, even when I select the slide show.
[Editor's note: I believe I also fixed this same problem once by simply trashing any preference file I found that had "screensaver" in its name. The above is a more elegant solution ;-). In addition, if you have a bad module that's causing the crash, this problem will continue to occur until you remove the offending module from ~/Library/Screen Savers.]

