A Strange problem. Please Help

Greetings,

I have a friend whom has a PC and he is using DSL the problem is when he shuts down his PC, about 5 seconds after it shuts down for some reason it boots back up without anyone touching anything. I am guessing it is wake on lan or something because when we unplug the DSL line it don’t do it only when its plugged in. Any idea on how we can stop his PC from turning back on when we shut it off.