There are six car-rental companies at Helsinki Airport: Avis, Budget, Enterprise, Europcar, Hertz and Sixt. Their service desks can be found in the arrivals hall of terminal 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.