If you're using zipped files make sure that you're using the 128 or 256-bit AES encryption available in the later versions of WinZip (or other encrypting zip utilities, there are several free out there).
The ordinary traditional WinZip 2.0 encryption is NOT strong and there are dozens of utilities (many of them free) out there that can break it without raising a sweat. It's only there to keep out casual snoopers.