FreeOTFE for Windows

FreeOTFE is a free, open source, "on-the-fly" transparent disk encryption program for PCs and PDAs
Using this software, you can create one or more "virtual disks" on your PC/PDA. These disks operate exactly like a normal disk, with the exception that anything written to one of them is transparently, and securely, encrypted before being stored on your computer's hard drive.