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Hallo...

Most likely it's either a corrupted copy of Windows on the disk or the hard drive itself is bad. You'll have to re-install Windows either way; if there are problems getting Windows loaded on the current hard drive then it's time to replace the hard drive.
 
First you can try booting to the Windows CD and running the Repair option to see if that gets you up and going. If that can't even find the disk, then I'd replace the drive.