I'd make one change...do the voice over while filming your video. I think it would be easier to get out the points you are trying to make while filming the video than rethinking what you were trying to get across later in ediiting. Then, as Tim suggested, you can use Windows Movie Maker which I believe will allow you to edit out 100% of the existing audio (but use the audio for a guide for the new audio), then record a clean audio track over when you have it fine tuned. Window's Movie Maker is pretty flexible and not difficult to learn to use adequately.