If securing by means of a password does not suffice, you might consider to encrypt your data by means of encryption software. By coding the data, your files will become unreadable to anyone who does not have the correct encryption key. You may code an individual file, but also (part of) a hard disk or USB stick.
Instead of encryption software, you could also use a ZIP program, such as WinZip or 7-Zip. ZIP software is meant to compress files, so that they take up less space, but it also has a strong encryption option.