Monstroso (Pocket Money Puffin) Charlie Higson (Paperback)

Oscar is not looking forward to Grandad coming to look after him at the weekend - it means he'll have to play endless rounds of chess or Monopoly, instead of war games on his computer. But when he finds Dad's ancient laptop in the study, he can't resist logging on to something called Project X - and all too soon Oscar creates a real live warrior.