Protect your data from loss, theft, or hackers

Improved for Windows 7 and available in the Ultimate and Enterprise editions, BitLocker helps keep everything from documents to passwords safer by encrypting the entire drive that Windows and your data reside on. Once BitLocker is turned on, any file you save on that drive is encrypted automatically.

BitLocker To Go—a new feature of Windows 7—gives the lockdown treatment to easily-misplaced portable storage devices like USB flash drives and external hard drives. 

source :http://windows.microsoft.com/en-AU/windows7/products/features/bitlocker