I am a noob myself but i think:
a solution could be
creating a transparent fullscreen gui that is always on top
maybe like
Code:
Gui, +AlwaysOnTop +LastFound +Owner +Toolwindow
Gui, Color, %CustomColor%
Gui, -Caption
WinSet, TransColor, %CustomColor% 0
Gui, Show, x0 y0 Maximize
(see "Example: On-screen display (OSD) via transparent window:" on
http://www.autohotkey.com/docs/commands/Gui.htm for a start)
and using the onmessage() command to receive all Windows messages related to keypesses with WM_KEYDOWN and WM_SYSKEYDOWN.
returning a var with this should terminate all these messages so the Keystrokes won´t fall through to outher programms or the system.
(see "What the Function Should Return" and "Example: The following is a working script that monitors mouse clicks in a GUI window." on
http://www.autohotkey.com/docs/commands/OnMessage.htm for a start)
you could let the funtion called by OnMessage() add each incoming char to a string and then check if this string containes your secret word.
(you should probably empty this string, when it exceeds a certain size)
but it´s just a guess