The Eternity Code by Eoin Colfer

Thirteen-year-old criminal mastermind Artemis Fowl has constructed a supercomputer from stolen fairy technology. In the wrong hands it could be fatal for humans and fairies alike. But no need to worry, Artemis has a brilliant plan. He's not going to use the computer; he's just going to show it to a ruthless American businessman with Mafia connections. His bodyguard, Butler, will be with him. What could possibly go wrong...?