To decrypt encrypted files, you will need an unzipping program that supports decryption of zip files that use 256 bit AES encryption.
We use WinZip V9, but others are available.
Examples are:
1) 7z. See
www.7-zip.org. Free. XP/Vista.
2) TUGZip. See
www.tugzip.com. Free. XP, but not Vista (tested Feb 2009).
3) WinZip (V9 or later). See
www.winzip.com. Not free.
4) StuffIt (Usually Mac but also available on PC). Not free.