Here are the symptoms: open System Preferences, select Desktop & Screen Saver and click on the Screen Saver tab. In the list of the screen savers, I had an RSS feed which turned out just to be too slow for my aging machine.
However, selecting the feed to delete it just gave me the beach ball, rendering System Preferences totally unresponsive. I gave up after a few minutes. To close System Preferences, click long on its icon in the dock and select "Force Quit".
To delete the RSS feed, I resolved to tracking down the corresponding preferences file and I found this one:
While System Preferences is closed, open the file in a text editor (I like TextWrangler) and delete the feed entry from the dictionary. My resulting file looked like this:
Save the file and reopen System Preferences to verify that the RSS screen save is no longer in the list. Done!
Update: I hear that you also can simply delete the file and you may need to restart or at least logout and login again.