The GoPro HD records .MP4 videos. These videos use the avc1 video codec and mp4a audio codec. The easiest way to get media player to play these is by installing the K-Lite codec pack.
You can download the codec pack here:
If you're running a 64-bit version of windows, you'll also need to install this one (please install BOTH - media player is 32-bit and movie maker is 64-bit):
Once this is installed, try playing a recorded .mp4 file using media player. When you play it, you should see some new icons appear in the tray:
If at this point the video is not playing, try:
(Something else may be trying to decode the video)
Now you should be able to play the videos in media player, and import and play them in movie maker.
Note: if you're using Windows XP (Movie Maker 2.1), you may have to go to Tools–>Options, and enable the ffdshow filter under the Compatibility tab. After you've done this rename the GoPro video to .avi and it should open in movie maker.
Now that you've imported and edited your video, you need to publish it without loosing a lot of quality. To do this you'll need profiles created that match the resolution an framerate of the recorded video. I have created these profiles which you can download and use. If you'd like to create you own, install windows media encoder, and use the Profile Editor tool it gives you.
Now when you publish your movie, you'll see the GoPro HD profiles. Choose the profile that matches your recording settings: