Problem with iTunes and Screensaver

Since a little while i can't maximize iTunse when it is minimized. When i click the green maximize button iTunes stops playing and quits. (At this moment i have it running minimized)
I can maximize it when i use the button "store" - "show my account", but when i want to go to my music iTunes freezes and quits.
At the same time, i can't use my screensaver anymore, if have selected in the past "flurry" as screensaver. When i want to start my screensaver via a interactive corner the screensaver starts but inmediatly freezes and quits.
I tried to select a new screensaver in "Preferences" - "desktop/screensaver" but i can't get into the "screensaver" field to change it. Than the preference pane freezes and quits.
Does anyone have an idea what to do?
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In addition to the post above.
Time Machine isn't working either. I have configured Time Machine and it does back-up my system as it should, but when i want to start Time Machine to take a look at my back-up it freezes and causes the Finder to restart.