Recorded video visible on device, but not other media players once uploaded

When I record videos with the rear-facing camera on my Archos 43 Internet Tablet, I am able to watch those recordings on my device, but when I attempt to upload them to online video sharing sites like youtube or blip.tv, they aren't properly recognized or converted. Likewise, when I transfer them to my computer, I can't play the video file correctly. If I transfer the file back to my device, it plays just fine. I know that the device produces *.mp4 video files. How can I convert these (corrupted?) videos so that I can see them on my computer and upload them to video sharing sites? Thanks.