BitLocker™ Drive Encryption (BDE) provides stronger security on Windows Vista Systems even when the system is in unauthorized hands or is running a different or exploiting OS. BitLocker™ Drive Encryption ensures boot integrity by protecting the system from offline software-based attacks, protects data while the system is offline by encrypting data and system files including user data, system files, page file, hibernation file, and temporary files and eases equipment recycling. BitLocker™ Drive Encryption is designed to provide a transparent user experience that requires no interaction on a protected system.

More information about Bitlocker can be found at:   http://www.microsoft.com/whdc/system/platform/pcdesign/secure-start_exec.mspx