BitLocker is Microsoft’s volume encryption solution built into several versions of Windows since Windows Vista. (There is an excellent Open Source alternative in TrueCrypt, but it just has that 3rd party feel to it. That and this particular task is easier with TrueCrypt so you wouldn’t need my help)
Normally, BitLocker use the TPM (i.e. Trusted Platform Module – see Wikipedia) to use your physical hardware as a factor in the encryption key, and leverages the key-toting abilities of the TPM at boot time. So long as your hard drive remains inside your machine it can boot. Take it out, and it’s just a mess of encrypted junk (Although your backup key can still unlock it).
I’m guessing that if you landed here, you are keenly aware of Bitlocker and it’s uses. You may have already tried to use BitLocker on a Virtual Machine and failed. This article is for you. – read on!