Your hardrive has probably crashed. I have been trying to fix mine and after weeks of studying, i found out I need a new video card. If you have demos then my problem may be yours too. If you are trying to play games and it crashes, then it's the video card. The only game that works with a broken video card is Street Fighter 4, but you can't save.
Visit this link for better instruction on how to fix non-video card problems. I hope it helps!
thx
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