Well, I think it's safe to say I'm in over my head with all this zlib stuff
I see what you mean about the DEF'd files not showing any signs of the original file though.
Perhaps it'd be best to, as MojoMan suggested, go to somewhere more generally experienced with this kind of thing. I hope you're right in guessing that it's zlib though, because if it turns out it's not, that kinda leaves us with no idea whatsoever of how to un-DEF them, doesn't it?
EDIT: Looking around that zlib website though, it doesn't look like zlib provides encryption. So if it is
zlib it's just a matter of getting it to de-compress, which by the looks of it you have to use the DLL files there which would involve writing a program to do it I suppose (I tried searching for one but didn't come up with much, I suppose zlib can't be a very common encryption method for files - probably used more for internal stuff with a program, like sending messages and stuff, since it doesn't have the ability to store information about the files in its header). But if it's compressed AND encrypted (or just encrypted) we could be in for some fun