I have not found any very good free software to edit on a PC, Herblenny is right. Quicktime and VLC are your best players. you should be able to play it in media player if you download and install the codec
http://www.free-codecs.com/download/K_Lite_Mega_Codec_Pack.htm I have confirmed that this works and plays .mp4 in windows media player.
for a PC paid solution download
Sony's Vegas Movie Studio
If you are a PC user and want the free solution.
1) Download
Any Video Converter - It will convert the MP4 to a WMV.
2) If you are a WinXP user, download the plugins (
http://www.papajohn.org/Downloads/) and install so it will write in 1280x720 format. Save the plugins in the C:\program files\Movie Maker\shared\profiles (if the directory doesn't exist, create it).
3) Use Windows Movie Maker to make your video.
4) Save and/or post to mtbr and enjoy!
I have not tried this yet but have heard the conversion quality is pretty good for a free product.
I still have a few GoPro Cams in Stock, PM me if you would like one!