UPDATE: Added link to the program here. It's beta and doesn't yet have all the super-secure (if the adversary is prepared and knows what he's doing) features discussed here, but it works.http://www.mediafire.com/?wtgywzo2znt
Edit the file to set the timer values. The key combination to reset the timer is Win+3.
You also need to have http://www.autohotkey.com/
Original code that has a different timeout (fake) warning (task tray balloon) below.
Use the computer with full confidence in situations where it can be snatched (stolen) from your hands. Any public areas actually qualify - and even your (g/b) friend, parent, teacher, bouncer, security guard, police, may just choose to take a look. Or if you absentmindedly leave your system unsupervised for a while, they can't get a (long) peek. The more portable things get, the more easily you lose physical control of them. The solution: you need to provide a "heartbeat" to the computer. They'll get the body but not the mind...
Send keystroke, activate screen saver, blank screen, display an image (favorites: black screen with "Missing Operating System", or "NTLDR is missing", or "Operating system not found", actually switching to DOS graphics mode or the trusty old BSOD), run video, run program, shut down, etc. unless a key is repeatedly pressed within, say, 10 sec. Could be one time only, e.g. during startup.
If you fail, it will optionally ask for a (probably quick and easy to type) password or a (modifier combo) key (or no dialog, you'll just type it) and if you fail to provide it in, say, 30 seconds, it will send a keystroke which will e.g. forcefully dismount encrypted drives.
Optional visible countdown timer with custom position, font, color, transparency. It could stay invisible until 5 sec remain and then change color from green through amber to red, or become more opaque and start blinking when 3 sec remain. If wrong key is entered, the color will optionally change, and optionally a sound will play (lowest priority feature).
Use a string of characters that has to be entered in order, repeating, or any one of them. So e.g. you'd keep on pressing Ctrl+Win+A, Ctrl+Win+S, Ctrl+Win+D to keep the system happy.
The key can be anything, with a modifier or modifier combo or no modifier.
Could support the finger print reader, microphone input (one of a group of recorded sounds/words).