PLEASE ANSWER ASAP!!!! EASY 10 PTS?

I have to do a project for a computer cl that I am in. In it, I must create a problem and the instructor has to find it. The one who creates the best and hardest to find problem wins a new CPU. It can be for mac OS X 10.4.11 or Windows 98. I just need someone to tell me how to make a virus or something that is very discrete. Thanks