There are hidden files and folders that you have not seen.
Windows by default reserves 10% of the hard disk space for the recycled bin, represented by the C:\Recycler folder.
System Restore also reserves 10-12% (I think) for the C:\_Restore folder.
Then there's the virtual memory, the minimum set at 1.5 times your RAM memory, maximum 3 times, represented by the file C:\pagefile.sys
There are other boot or system files in the C root directory, but they are very smalll in size.
They may still not add up to the expected figure, but you can never quite work out exactly how Windows does the calculation, and whether the reserved space (rather than actual space used) is taken into account.
At least the Recycler, _Restore, and pagefile.sys files and folders will account for some of the apparently missing space.
To see the hidden files, in My Computer, click Tools, Folder Options, View tab, tick Show hidden files and folder, and untick Hide protected operating system files, and click OK.