TrueCrypt is a free open-source disk encryption software.
Creates a virtual encrypted disk within a file and mounts it as a real disk.
Encrypts an entire partition or storage device such as USB flash drive or hard drive.
Encrypts a partition or drive where Windows is installed (pre-boot authentication).
Portable TrueCrypt can be run from a removable drive on any Windows computer without installation.
Download: Portable TrueCrypt 6.2a