To unzip files that are zipped, you will need a suitable unzipping program.
We use WinZip V9, but others are available.
Examples are:
1) 7z. See
www.7-zip.org. Free. XP/Vista.
2) WinZip (V9 or later). See
www.winzip.com. Not free.
3) StuffIt (Usually Mac but also available on PC). Not free.