Windows Media Player is very limited when it comes to the types of files it can play.
Download the excellent, free VLC Media Player and try playing your file with it. I think you've definitely got an MPEG there.
Sweet that did the trick thanks!
but I really want to use mpeg for the HD quality.
I have to disappoint you, the mpeg used is SD quality as in DVD quality.
If you want to play your gopro footage (which i persume is in HD) use a mp4 1920x1080 preset for playing the movie on PC.
Thanks I'll try that!