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?

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.