I deleted the preferences plist from the library folder and from the user folder but the problem is still existing. I also tried it with an different user but without success. It always looks like this when I create or open a document:

http://www.imgimg.de/uploads/iwork41e83449jpg.jpg


Edit:

I also found out that my screensaver lags and iTunes doesn't show my cd covers. But my graphic card (7900GT) is correctly recognized.