There are six car-rental companies at Helsinki Airport: Avis, Budget, Enterprise, Europcar, Hertz and Sixt. Their service desks can be found in the corridor between terminals 1 and 2. You can pick up the rental cars easily from the parking hall P3.
You can pre-book a rental car by calling to the car-rental dealership or from their websites. You can also hire a car when at the airport. There is a variety of rental cars to suit every taste.