There is in Windows file systems a pre computed hash for each file?
Windows does not store a hash for each file. As Jader Dias suggests, there are checksums for EXE's and DLL's but these are not the droids you are looking for.
Note that even if you had such a hash, it still does not guarantee uniqueness. If you found two files with the same hash (and size) you would still have to then compare contents to determine if the files were truly the same.
JPEG files may have some checksums or hashes, but you probably cannot count on them either.
Windows though does have search now & if I recall correctly you can write your own plugins for it (in other words, to index files in a custom way). Presumably, you could write a plugin for JPGs & then simply make search API calls to find files (after Windows does the indexing).