Opey basically summed it up. With the bios password, he cannot get past it unless he opens up the case and either pulls the jumper to clear the cmos or he yanks out the cmos battery... I've done both before... Of course if the cmos is always being cleared, it'd get suspicous... I once ended up figuring out what keys were typed for the password and figured out the password that way..
Some computers have a way for you to lock the case with a lock or a key. Do that and you are pretty much set.