This can be difficult - been posted in the past. I believe Quicktime Pro (paid for version) can do it. Also Adobe Premiere Elements video editing package may be able to do it.
made the same silly mistake myself. I have tried for ages to find freeware/shareware that would do this but without any luck. The quality of the video doesn't justify investment in serious software - I just treat it as a lesson to remember (if I am sober at the time)