Using Firefox, Thunderbird and XP.

I open my emailer throughout the day, many times. It is used for business and pleasure. Is it possible to require a password before it will open? I do not need something 6 or 8 digits long and designed to prevent access by a hacker. I want something very, very simple, so as to discourage the person casually passing by, while I am away from my desk.

I do not want to encrypt messages. I want to click on the email icon and have it ask for a one- or two-digit password.