TrueCrypt is a free open-source on-the-fly encryption software for Windows XP/2000/2003. It can create a virtual encrypted disk within a file and mount it as a real disk. It can encrypt an entire hard disk partition or a device, such as USB memory stick, floppy disk, etc. It is the only free open-source on-the-fly encryption software that fully supports Windows XP. It is also the only free open-source OTFE software for Windows XP that provides plausible deniability. Encryption algorithms: AES (256-bit key), Blowfish (448-bit key), CAST, IDEA, Triple DES Based on Encryption for the Masses (E4M) 2.02aTrucrypt is the best free open-source on-the-fly encryption.