Though many people already have a login password for wondows on their PCs, it doesn't mean that all your data is protected. The reason is that should someone steal your computer then what he'll have to do is simply remove the hard drive and plug it to another computer.
To be able to fully protect your important data getting into wrong hands, you can encrypt your hard drive such that even if it's stolen, the data still remain safe.
Microsoft's Bitlocker software makes setting up full-disk encryption in windows incredibly easy as long as your computer has ultimate or Enterprise version of windows 7 or vista or the pro, or Enterprize version of windows 8. And as long as your computer has TPM(Trusted Platform Module) chip.
To enable BitLocker, go to Control Panel > System and Security > BitLocker Drive Encryption, or do a search for "BitLocker" in Windows 8. In the BitLocker menu, click Turn on BitLocker next to the drive(s) you wish to encrypt. It's as easy as that.
If your PC doesn't meet the requirements for BitLocker, you can still use TrueCrypt or DiskCryptor for free full-disk encryption.
source:pcworld (http://www.pcworld.com/article/2025462/how-to-encrypt-almost-anything.html)