What's happen,if we deleted all empty files in 'C' , 'D' and Registry.?

deleting registry files is dangerous, even if they have no values in them. provided there are no hidden folders in c and d it probably wouldnt cripply your computer, but you may lose some functionality.

many different kinds of software create empty folders as a place to put temporary files for use while you are using the program, and once you close it, the files are deleted.

What do you mean by empty files? If you mean you open them and there is nothing in them , no problem. Be very careful with the registry, removing an apparent empty entry can impact on other entries.

Nothing at all (that's if there are no hidden files within)