TrueCrypt

TrueCrypt is a software application used for real-time on-the-fly encryption. It can create a virtual encrypted disk within a file, or a device-hosted encrypted volume on either an individual partition or an entire storage device. It supports Microsoft Windows, Mac OS X and Linux, and encrypted volumes can be made portable. The version for Windows Vista or XP can encrypt the boot partition or entire boot drive, and has the ability to create and run a hidden encrypted operating system whose existence is deniable.
Portable TrueCrypt Freeware