the files step a 5 the operation of the embodiment will be explained in more detail by way of an example for instance the computer 12 is supplied with the software installed therein the storage medium 121 is a hard disc drive built into the computer the software installation system 11 is to reset the computer 12 to the initial state as was delivered to the user the contents in the storage medium 121 in the computer 12 was changed by the user the storage medium 114 in the software installation system 11 stores the files which were initially stored in the storage medium 121 when the computer 12 was delivered to the user as shown in fig3 the storage medium 121 in the computer 12 stores files a b d and e the storage medium 114 stores files a b c and d the file operation determination device 112 is instructed such that u201c no file operation is performed when the files in the storage medium 121 and the files in the storage medium 114 have no difference u201d u201c the file is copied from the storage medium 114 to the storage medium 121 when the file in the storage medium 121 differs from the file in the storage medium 114 u201d u201c the file is deleted from the storage medium 121 when the corresponding file is not stored in the storage medium 114 u201d and u201c the file is copied from the storage medium 114 to the storage medium 121 when the file in the storage medium 114 is not stored in the storage medium 121 u201d the difference detector 111 compares the files in the storage medium 121 with the.