Free open source on-the-fly encryption software
In Windows, a user who does not have administrator privileges can use TrueCrypt, but only after a system administrator installs TrueCrypt on the system. The reason for that is that TrueCrypt needs a device driver to provide transparent on-the-fly encryption/decryption, and users without administrator privileges cannot install/start device drivers in Windows.
After a system administrator installs TrueCrypt on the system, users without administrator privileges will be able to run TrueCrypt, mount/dismount any type of TrueCrypt volume, load/save data from/to it, and create file-hosted TrueCrypt volumes on the system. However, users without administrator privileges cannot encrypt/format partitions, cannot create NTFS volumes, cannot install/uninstall TrueCrypt, cannot change passwords/keyfiles for TrueCrypt partitions/devices, cannot backup/restore headers of TrueCrypt partitions/devices, and they cannot run TrueCrypt in ‘portable’ mode.