Are you suspicious that someone is using your computer without your consent? Or do you worry what others are using your computer for? If so, then this program could help you get to the bottom of any suspicious activity on your PC. Once activated, it automatically detects any keystrokes made on your keyboard including passwords and IM conversations. It fulfills all the criteria you need from software like this - it's light, discreet and very easy to use.

The program runs quietly in the background and there's nothing a user can touch on the keyboard to make the program appear suddenly. Once installed, you click on the red button in the toolbar to start monitoring. To test it, open an application (such as Notepad), write some text in it, press "Enter" and the text entered will appear in the main Revealer window. You can then hide it by clicking the eye button of the toolbar. To make the window visible, just press the hot key (the default is Ctrl + Alt + F9) or launch the executable file.

This is pretty much the most discreet program I've used for this sort of thing. The only way someone can see if this program is running is by going into the process list of the task manager. However, that's not exactly difficult and if only that were fixed, this would be an excellent spying program.