BitLocker is a piece of software by Microsoft that encrypts your hard drive and prompts the user for a password if Windows detects a hardware change. This helps to protect your data if your computer is stolen.
To install BitLocker on a Windows Vista computer you must first have Windows Vista SP1 installed. Next download and run the Bit Locker Drive Preparation Tool for Windows Vista, before running BitLocker from the Start Menu. After installing BitLocker, the drive will be encrypted over time (you can still reboot and install apps while this is happening), this can take some time. Once BitLocker is installed, I recommend running Windows Update and installing any recommended updates for Windows Vista and BitLocker.