BitLocker Drive Encryption is a data protection feature of Vista which will be only available in Windows Vista Enterprise and Ultimate editions
SecureAction has released CryptoExpert 2007 Lite edition, a freeware software for Windows Vista 32-bit and 64-bit that provides similiar functionality as BitLocker Drive Encryption and allows you to create Virtual hard disks with 128-bit encryption and drive letters on your computer. The files in the drive can be accessed as if from a regular hard drive.

Main features of this application include:

-Using powerful encryption algorithm to encrypt your data: CAST 128 bit
-Ability to create encrypted containers (files vaults) up to 20 Megabytes in size
-Access to your files on virtual volume on the fly
-Mapping any free drive letter like G:, F:, K: for virtual volume
-Requires password authentication before the files become accessible
-Works with all Windows application - even DOS applications

Download here