Revealer Keylogger Free is the free edition of a commercial keylogger.
The developer claims the program can capture everything typed at the keyboard in any application.
There are other free keyloggers around, but what's interesting here is that the program can be set up to run on startup for specific users only. You could configure the system to record keypresses on your kid's accounts, for instance, but not yours.
A customisable hotkey opens the program when necessary. It's password-protected to make it harder for other users to disable. You can view the logs, and there's an option to have the logs automatically deleted after a specified number of days.
A £29.99 commercial version extends the package with automatic screenshots during the capture session, "invisible mode" to hide the program in Task Manager, at startup and on the drive, and the ability to send the log files to a remote user via email, Dropbox, ftp or the local network.