just to clear it up for you, a TPM is a Trusted Platform Module. When you use bitlocker, it stores the keys that are required to access the hard drive in the TPM. If the hard drive is taken out of the computer, it won't work because the TPM is not there.
Bitlocker is used as a complement to login passwords and EFS (encrypting file system).
Take this scenario: you have a logon to your computer, but not bitlocker. Someone boots up the computer, but can't get through the logon. No problem, they take the drive out of the computer and put it in another as a second drive- bingo, data is there.
With bitlocker turned on, when the drive is removed from the computer it is completely encrypted and cannot be accessed.
The bitlocker in windows 7 ultimate uses either a TPM or a USB (which needs to be plugged in on boot to access the keys) to encrypt drives. Windows 7 makes it alot easier to set up, its mainly automatic. Vista required manual partitioning to set up properly.