BitLocker™ Drive Encryption addresses the very real threats of data theft or exposure from lost, stolen or inappropriately decommissioned PC hardware with a tightly integrated solution in the Windows Operating System.  BitLocker prevents a thief who boots another operating system or runs a software hacking tool from breaking Windows Vista file and system protections or performing offline viewing of the files stored on the protected drive.  This data protection feature is available in Windows Vista Enterprise and Ultimate.

 

http://www.microsoft.com/technet/windowsvista/security/bitlockr.mspx