Learning to play the piano is a prolonged challenge for novices. It requires them to learn sheet music notation and its mapping to respective piano keys, together with articulation details. Smooth playing further requires correct finger postures. The result is a slow learning progress, often causing frustration and strain. To overcome these issues, we propose P.I.A.N.O., a piano learning system with interactive projection that facilitates a fast learning process. Note information in form of an enhanced piano roll notation is directly projected onto the instrument and allows mapping of notes to piano keys without prior sight-reading skills. Three learning modes support the natural learning process with live feedback and performance evaluation. We report the results of two user studies, which show that P.I.A.N.O. supports faster learning, requires significantly less cognitive load, provides better user experience, and increases perceived musical quality compared to sheet music notation and non-projected piano roll notation.