Tip Case

TrueCrypt can create a virtual encrypted disk within a file and mount it as a real disk, or an entire hard disk partition or a device, such as USB memory stick, floppy disk, etc.

This software provides two levels of security: a hidden volume (more information may be found here), and a volume cannot be identified (no part of the volume can be distinguished from random data).

It utilizes the following encryption algorithms: AES (256-bit key), Blowfish (448-bit key), CAST5 (128-bit key), Serpent (256-bit key), Triple DES, and Twofish (256-bit key), and also supports cascading (e.g., AES-Serpent-Twofish).
It is based on Encryption for the Masses (E4M) 2.02a, conceived in 1997.

If you are looking for a light freeware for protecting and encrypting your important files, I think FileEncrypt is the one.

FileEncrypt is a tiny and handy program to encrypt your important files by a strong and fast encryption algorithm. What you need to do is type your password and select a file. Also the advanced key strong length feature is included. When you want to decrypt your file, just put your password into the password field and click Decrypt button to select the encrypted file, easily and simply.