This prelude is from Johann Sebastian Bach's The Well Tempered Clavier, or The 48 Preludes and Fugues. The prelude should ideally be played at a slow pace, with the quavers forming a steady tempo. Bach's Preludes and Fugues come in matched pairs in the same key, and are often (though not always) played as a pair in the order Prelude first and then Fugure. Here is the corresponding Fugue for this Prelude, also in the key of B minor. A must for any serious student of piano, the complete Book 1 and Book 2 are available from Sheet Music Plus. The video below is a piano roll animation for the music, or you can play the mp3 file and see the sheet music score before downloading.