This is a typical problem when a computer crashes. What has occurred is that your PC was attempting to write a file when the crash occurred. It had written something but not completed the process by deleting an earlier version and up-dating the File Allocation Table (FAT). This results in sectors of the hard disk being recorded in the FAT as in use when in fact they are not. Typically, SCAN DISK simply deletes the unneccessary file fragments in the FAT, thereby restoring the space for future use - some versions convert the fragments into actual files so that you can control whether the information should be used or deleted.