You can use a hardware keylogger attached between the keyboard and the computer.
but that would beat the whole point of hacking won't it?
here is an easier way, but not guaranteed to work if your cousin is smart enough.....
code a warning application and put it in the startup.. something like
Alert!...
the system has encountered problems while booting up. Please verify your BIOS password for continued protection of the system.
provide a DOS screen, maybe use a simple bat, for some reason people are impressed when I work on a black and white window lol..
Yeah, I know it is lame but it really worked for me once
. It will work only if your cousin does not know too much about computers.