TrueCrypt

Free open-source disk encryption software for Windows XP/2000/2003 and Linux

TrueCrypt is a software system for establishing and maintaining an on-the-fly-encrypted volume (data storage device). On-the-fly encryption means that data is automatically encrypted or decrypted right before they are loaded or saved, without any user intervention. No data stored on an encrypted volume can be read (decrypted) without using the correct password/keyfile(s) or correct encryption keys. Entire file system is encrypted (e.g.., file names, folder names, contents of every file, free space, meta data, etc).

Read more:
TrueCrypt Introduction
TrueCrypt Wikipedia Page
TrueCrypt Tutorial

A very professional software to encrypt and decrypt entire drives with a collection of powerful encryption and decryption algorithms. The entire encryption and decryption is carried out on the fly and fast. Any file added to a encrypted volume also gets encrypted. It creates a virtual container very much like a hard disk drive in which everything once put is encrypted. This effectively means we can use it to encrypt and decrypt folders/files/drives or rather any data on our hard disk.

Windows XP/2000/2003
Direct Download Link: truecrypt-4.2a.zip