Of Spam & Men

Vista's Entreprise and Ultimate editions will introduce a new (for Windows) security feature called BitLocker:

BitLocker Drive Encryption is a data protection feature available in Windows Vista Enterprise and Ultimate for client computers and in Windows Server "Longhorn". BitLocker is Microsoft’s response to one of our top customer requests: address 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.

You can configure Vista to require an USB pen key or a flash drive to boot. If the user can't provide the right key, the data on the harddrive stays encrypted and (relatively) secure.