WinSxS
From Vista onward the operating system also uses WinSxS for its core components. Operating system files in the WinSxS directory are hard linked to their usual locations in the Windows directory structure. The same file may be linked from a directory in WinSxS and from, say, the System32 directory. Windows Explorer double counts the disk space occupied by these files. This can be demonstrated using the fsutil command-line program. (Some third-party Explorer extensions to show the link count also exist.) However not all files from WinSxS are projected this way to "live" operating system files. For example, after installing some Windows updates, old file versions replaced by the updates are still kept in WinSxS although they are no longer linked in the "live" Windows directories. This allows updates to be uninstalled safely.
Because of its elevated importance, from Vista onward the WinSxS directory is owned by the Trusted Installer service SID. By default not even administrators can modify its contents (without taking ownership first). Uninstalling applications does not immediately free space in the WinSxS directory; space for unused assemblies is garbage-collected over time by the Installer service.
Although not officially documented, the algorithm for generating the directory names residing inside the WinSxS directory has been made public on an MSDN Microsoft employee's blog. The algorithm was changed in the transition from XP to Vista.
Read more about this topic: Side-by-side Assembly