Encryption is the process of coding your files so that unauthorized parties cannot read them It's normally used to protect highly sensitive documents, but it's equally useful to stop people from looking at your personal stuff.
The favorite one is an Open Source program called AxCrypt. It provides secure AES-128 encryption using passwords or key-files, which AxCrypt can generate. Usage is wonderfully simple; to encrypt a file or folder, just right-click it and select "encrypt." Double clicking on an encrypted file lets you edit or view it with the program of your choice. Closing an open file automatically re-encrypts the file.
dsCrypt is another option. Here's what the website says: "dsCrypt is AES/Rijndael file encryption software with simple, multi-file, drag-and-drop operations. It features optimal implementation, performance and safety measures. dsCrypt uses an advanced encryption algorithm and offers unique options for enhanced security."
To that I'll add that it's tiny, easy to use, has open code, employs proven techniques that are essentially unbreakable and comes from a highly reputable source.
You can use another Open Source Program called TrueCrypt for file encryption, but its a more complex program and is better suited to encrypting a lot of files, folder and drives.
AxCrypt
Website: http://www.axantum.com/axcrypt/
Download link: http://www.axantum.com/axcrypt/
Author: Axantium Software Ltd.
Current version: 1.6.3
Version date: July 2007
Download File size: 1.10MB
License: Unrestricted use freeware
Operating systems supported: Windows 98-Vista
Additional Software Required: None
64 Bit version available: (beta only)
Portable version available: No
Non-English languages supported: None yet
Other relevant information: None
dsCrypt
Website: http://members.ozemail.com.au/~nulifetv/freezip/freeware/
Download link: http://members.ozemail.com.au/~nulifetv/freezip/freeware/
Author: nulifetv
Current version: 1.100
Version date: 2006
Download File size: 25KB
License: Unrestricted use freeware
Operating systems supported: Windows 98-Vista
Additional Software Required: None
64 Bit version available: No
Portable version available: No
Non-English languages supported: None
Other relevant information: None