BitLocker encrypts the entire Windows volume (in most cases, this is drive
C
. If your data is on the same drive, your data is encrypted as well.
BitLocker's goals are different than EFS's. BitLocker protects the entire
volume from offline attacks. If someone steals your laptop, removes the
drive, and mounts it in another computer, the information on your drive will
appear as garbage, because it's encrypted. You're right, if someone can
successfully log on to your computer, using *any* account on the computer,
then BitLocker won't help you there. This is true of all products that
perform volume or drive encryption.
EFS protects individual files. To access protected files, you have to log on
as the user who owns those files. If you log on as some other user, you
can't access the files.
A few months ago we released the Data Encryption Toolkit for Mobile PCs
(
http://www.microsoft.com/technet/security/guidance/clientsecurity/dataencryption/default.mspx).
It has a lot of good information explaining these technologies.