As was stated in a previous post in this thread, VirtualDub would be your best bet. I don't have the app in front of me but basically for video you want to choose the option which basically says "pass thru" or "no conversion"... those won't be the terms but it's something close to that. For Audio, you definitely want to convert to mp3. So, it will retain the video stream but convert the audio. This process should be very fast since you are only converting the audio (a minute or so). Still beats the pants off of having to convert the video from Xvid to H.264 (which will take potentially an hour+).
On a side note, I do agree that H.264 is the "modern" codec to use. I have started seeing sites start encoding stuff to H.264, but there are still a ton of videos in Xvid format.