Have you got any download, pictures, video or any other documents on your pc, also check how pany programs are installed and how much space they take (control panel--> add/remove program). If nothing, are you sure of the size of your drive? have you partitioned the drive? if still nothing, try to unplug and plug back to see if there is any change.
Last thing, try and delete the oldest backup files, they are useless if your PC was working fine on the last backup.
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