For exploring the Sacred Valley, a car is nice to be able to get around, but driving anywhere in Peru seems to have its own unwritten rules, so hiring a car and driver or going by taxi is the usually the safest option. Armando Aguero (974/213–172 or 953/724–887 email@example.com) is one such driver. He speaks Spanish, English, French, and Portuguese, and can connect with other drivers as needed.
The vehicular tourist route ends at Ollantaytambo. Cusco is the only place to rent a vehicle. However, you won't need or want to drive inside the city; heavy traffic, lack of parking, and narrow streets, many of them pedestrian only, make a car a burden.