I did something SORT of like that...
I used Cool Edit (though I think mine was only the demo version - it only did 60 seconds) and you open up the instrumental track and you can record yourself singing over the top on a different track. *realises she probably does not make a whole lot of sense*
Problem is, what I sang to was someone else singing along to some music :P So, for what it was the quality came out ok ^__^
