There are two parts to BitLocker drive exncryption in Vista, the Encryption software and the TPM chip. The software is available with Windows vista Ultimate Edition and the TPM chip is found on motherboards that specifically support that technology. Having a whole drive encrypted with BitLocker ensures that the drive can not be read by another pc and cannot be read without the password on your pc. The integrity of files and the system state is also monitored by BitLocker to ensure that the system has not been compromised.
The this form of encryption works is from when you boot up your pc.
