I've struggled with this one on and off: I was unable to use the backup/restore/zip/unzip files capability - always gave an error on writing new compressed or uncompressed files
I use php4.3.4 on apache, Win 2k server [zlib enabled automatically]
solution - delete the [UNIX only] paths to the zip and unzip progs, in the admin configure variables page
it seems that the windows version tries to use these anyway
Andy D