Which files must stay uncompressed?

[Stalkie]

I was just browsing through my i386-folder, and noticed some files that wasn't compressed.

Mostly DLL's, but I know that some of them can be compressed (Like WINNTBBU.DLL).

Is there somewhere og somehow I can find out which files have to be uncompressed, and which I can compress to save some space?

Some of the DLL's I figured have to be uncompressed, like NTFS-files and the keyboard-files.