Essentially, they are Windows files. For those whose copy of
Windows isn't too corrupted, you can boot up to DOS, then using DOS
commands to copy various files over to the Windows system
folder.
In XP, it will be Recovery Console.
In a way, they are backup copies of Windows files. They are
sometimes updated when Windows update.
There's also another folder, the Dllcache folder. This folder
contains all Windows DLL files, and can be copied over to the
Windows system folder the same way via DOS. Same thing, they are
updated when Windows is updated.
You can check whether it's modified by checking the folder
properties after you update Windows.
Also check out Windows KB site for details. They do list the files
they update Windows, together with the file sizes and version.