ME and XP OS's support unzipping, whereas all previous microsoft OS's do not. These two platforms almost certainly have some sort of DLL that controls the unzipping...but I haven't seen any documentation on it.

If you're proficient at c++ programming, you can modify GNU source code to create a dll that can zip/unzip. I've done an uncompress dll for unix Z files, and I think that code is available for the zip compression format from GNU also.