That song that Jack is improvising on is a bit of the theme song to an independent series he was a part of called "Computerman"
check this site out to listen to it!
http://www.channel101.com/shows.php?show_id=1
Hope that helped!
and also...does ANYONE know what the song is that they play during one of the final scenes in the movie? the one where the kids are waiting at the bus for Dewey??? It's got a heavy bass line and drum beat...sounds really punk-ish...This has been bothering me for quite some time and I stilll have NO idea what the song is! Maybe it was written especially for the movie- so even if thats true, does anyone still know where I came find a clip of it?