Truecrypt was one of the easiest full disk encryption programs for a very long time.  Better yet was the price – free.

The latest version allowed you to encrypt your entire operating system drive (Windows only) insuring anything you do would be encrypted. You could even make encrypted containers that would open on Windows, Mac or Linux machines.  I’m sure I have some encrypted containers somewhere in my backups so it is important to keep a link to the program available.