The problem with Bitlocker in Windows 7 (when encrypting system drives) is it'll literally do the whole drive, so if it 1TB in size it'll take hours.
All flavours of Windows 7 support Bitlocker. Starter, Home and Pro only support portable drives being encrypted. Enterprise will let you do system drives.
Windows 8's better in this respect - you can encrypt system drives, but it'll only encrypt the actual data vs. the whole drive. Considerably quicker and more intelligent.
It does do the encryption in the background though so can be used while it is doing it, even survives sleep and shutdown while its doing it.
With SCCM, you can encrypt the entire system drive prior to the operating system image being applied in WinPE. This enables you to encrypt a Windows 7 laptop really quickly since there isn't any data on the drive.
Originally Posted by Michael
Increasing the size of the "System Reserved" partition also helps to increase Bitlocker's encryption speed. I usually set it to 1GB.