Microsoft is working on Songsmith, a program that will take the tune you sing and make music to fit it.
The following demonstration of the technology needs no explanation:
The videos where people have fed classic tunes into Songsmith and recorded the results are just as "good." I like "Roxanne" and "Hotel California" myself.
In the words of Darth Vader, "Don't be too proud of this technological terror you've constructed."
(You know, I actually could imagine using this for a kind of musical notetaking ... but not for making an ad ...)