How to make simple error or popup messages in pc while my kid is gaming.(i want make him frustated)pls help.?

My friend (6 year) is playing games in pc continously.i want to stop it.so how to make some simple error messages or popup messages while he is gaming.i want to make him frustated.(give some way to make popup messages or simple error messages while gaming in Windows xp).(erors which won't harm my pc)