Have you checked First in Start Programs? I know on MY XP machine if a new program is installed it will show somewhat highlighted in the Program list. OR if you see a game you do not recognize just Right Click and choose properties, this will tell you when it was created on the machine.
Also check the internet history in your browser, this may give you a clue as to where he "surfed" and where he may have downloaded this game from.
Also, you might check Add/Remove to see which games are noted there. Just highlight each one and you should be able to see the installation date.