Please suggest a free editor to manipulate (re-record or dub) the audio in video files?

I suggest to use Windows Movie Maker. It comes with every XP or Vista computer. Go Start > Accessories > Windows Movie Maker to run it.

Windows Movie Maker is rather basic. But I hope that it will be enough for this task. With this tool you will be able to mute the original sound and add a new soundtrack.

To work with audio I recommend Audacity (free audio editor). You can download it here http://www.download.com/Audacity/3000-2170_4-10606824.html?tag=lst-0-1.

Some tips for Windows Movie Maker can be found here: http://www.microsoft.com/windowsxp/using/moviemaker/default.mspx.