This music is the Prelude in C minor (BWV999) by Johann Sebastian Bach. It is a stand alone work and often called the Little Prelude in C minor to distinguish it from the keyboard preludes in his Well-Tempered Clavier. However it is easy to imagine this little prelude as belonging to the same set. It was originally composed for a keyboard instrument called the "Lute-Harpsichord" but it is playable on any keyboard instrument including piano or harpsichord. Although it starts off in C minor the music quickly adopts a long D pedal and ends firmly in the key of G minor. On mfiles we also have an alternative version of this prelude for Guitar though transposed to D minor to better suit that instrument.
The video below shows the keyboard version of this Prelude being played on the Piano. Below that you can scroll down to play the mp3 file and see the sheet music. Downloads of the PDF sheet music, mp3 file and midi file are available via the links in the left-hand menu.