Free Disk Encryption Software TrueCrypt

Free open-source disk encryption software for Windows 7/Vista/XP, Mac OS X, and Linux
TrueCrypt is one of the most popular Open Source disk encryption software programs for the Windows operating system
It can be used to encrypt entire hard disks, partitions and to create encrypted containers on hard disks that have a specific size defined by the user. Some of the latest features that have been added include operating system encryptions and complete system encryptions. The latest version of TrueCrypt brings full support for Microsofts Windows 7 operating system that has been released today.

https://www.truecrypt.org/