Looking for help again....

Anyone know of any software that mimics either a piano keyboard or guitar fretboard? I.e. what I want to be able to do is click on either a piano key or a guitar fret and it plays the note through the 'puter's speaker.

Reason for asking is that my wife is learning to play flute and plays only from sheet music, while I play guitar entirely by ear,. While sitting on the train on my daily journey to the office, I'm trying to put down the melody that I want her to play in traditional notation in a PC notation package (Finale Guitar) on my laptop. The idea is that I can 'play' the melody using the mouse on the virtual keyboard / guitar neck, then transcribe the notes into sheet format.

If I was at home, I would be sitting there with guitar, working out the melody a few notes at a time, but I don't think other commuters would be particularly impressed if I sat there on a crowded train alternating between picking up the guitar and playing a few notes, then putting the guitar down to transcribe into the laptop!

Any help or other ideas would be appreciated!