To make it almost impossible to open without a password use either TrueCrypt or AxCrypt. I prefer TrueCrypt.
TrueCrypt is also strongly recommended for securing company confidential information on laptop computers. It will protect the information even if the computer is lost or stolen.
If you use a long password that contains letters, numbers, and symbols (preferably a generated password) it will be impossible within your lifetime to open the encrypted files without the password Do not lose your password!
In case an adversary forces you to reveal your password, TrueCrypt provides and supports two kinds of plausible deniability:
1. Hidden volumes (for more information, see the section Hidden Volume).
2. It is impossible to identify a TrueCrypt volume. Until decrypted, a TrueCrypt volume appears to consist of nothing more than random data (it does not contain any kind of "signature"). Therefore, it is impossible to prove that a file, a partition or a device is a TrueCrypt volume or that it has been encrypted.
Videos on how to use.
On-the-fly encryption software which can create a virtual encrypted disk within a file and mount it as a real disk. It can also encrypt a hard disk partition or storage device, such as USB flash drive. Moreover, TrueCrypt supports plausible deniability.
TrueCrypt provides two levels of plausible deniability, in case an adversary forces you to reveal the password: a hidden volume (steganography- more information may be found here); no TrueCrypt volume can be identified (volumes cannot be distinguished from random data). TrueCrypt uses encryption algorithms AES-256, Blowfish (448-bit key), CAST5, Serpent, Triple DES, and Twofish. It is based on Encryption for the Masses (E4M) 2.02a, conceived in 1997.
AxCrypt 1.6.3 (free: donation optional)
AxCrypt File Encryption Software - Free Personal Privacy and Security for Windows 98/ME/NT/2K/XP with AES-128 File Encryption, Compression and transparent Decrypt and Open in the original application.
Since AxCrypt works on a file-by-file basis, you can safely and easily send encrypted files to other users via e-mail or any other means, you can even make self-decrypting files so the recipient does not need AxCrypt installed to decrypt.
Add password to in XP a compressed file or folder.
(Vista does not support this)
Right click on the file or folder, choose send to compressed file.
Double click on the compressed file to open.
On the menu bar click on File and then "Add a Password"
Delete original uncompressed file or folder.
This password can be broken.
Another method to create a compressed locked files is to use IZArc or 7Zip. They both support 256-bit AES encryption which is much stronger than the password protection offered by the Windows built in ZIP utility.
IZArc supports 256-bit AES encryption to secure your data.
If you have broken archives IZArc can help you to repair them with ease.
IZArc is the ultimate freeware archive utility supporting many archive formats like: 7-ZIP, A, ACE, ARC, ARJ, B64, BH, BIN, BZ2, BZA, C2D, CAB, CDI, CPIO, DEB, ENC, GCA, GZ, GZA, HA, IMG, ISO, JAR, LHA, LIB, LZH, MDF, MBF, MIM, NRG, PAK, PDI, PK3, RAR, RPM, TAR, TAZ, TBZ, TGZ, TZ, UUE, WAR, XXE, YZ1, Z, ZIP, ZOO.
IZArc: How to split large archive
7-Zip supports encryption with the AES-256 algorithm. This algorithm uses a cipher key with a length of 256 bits. To create that key, 7-Zip runs a derivation function based on a SHA-256 hash algorithm. A key derivation function produces a derived key from the text password entered by the user. To increase the cost of an exhaustive search to crack a password, 7-Zip uses a large number of iterations when it produces a cipher key from a password.
Packing / unpacking: 7z, ZIP, GZIP, BZIP2 and TAR;
Unpacking only: RAR, CAB, ISO, ARJ, LZH, CHM, Z, CPIO, RPM, DEB and NSIS. For ZIP and GZIP formats (includes GZ)
High compression ratio in new 7z format with LZMA compression provides compression ratio that is 2-10 % better than ratio provided by PKZip and WinZip.
Password protect Microsoft Office files
Open file. On the Tools menu, click Options, and click the Security tab. Choose password options listed.
Demo of how to password protect Microsoft Office files
Set a password to open or modify a document, workbook, or presentation
Applies to: Microsoft Office Excel 2007, PowerPoint 2007, Word 2007
How do I password protect my files and folders in Windows?
How to make files and folders private in Windows XP so that only you have access to them
Windows Tips: Password-Protect Your Sensitive Files and Folders
Keep Your Data More Secure
Encrypt files or folders