File System Encryption for Windows (Free)

EFS (Encrypting File System)

EFS aviable with windows provides file encryption for free and works transparently with the os.

1) Open Windows Explorer and select the file you want to encrypt
2) Right-click the chosen file and select Properties from the context menu.
3) Select the Advanced button to enable EFS.
4) Encrypt the file by selecting the Encrypt contents to secure data check box as shown in Figure below. Click OK.

http://technet.microsoft.com/en-us/library/bb457065.aspx