Problem with MP4s in Live Movie Maker? Running Vista?

I'm trying to use MP4 video files in Live Movie Maker on my laptop that runs on Vista. Whenever I click the button that says "Add Videos and Photos", the video appears in the timeline but is completely grey. When I click play, it shows that I'm watching the video but it's completely grey and there's no sound. How do I get this file type to work? I have tried using converters, but they all either leave watermarks, or are just trials. Do I just need to install some codecs? If so, where do I get them? Please help me!