A tip for you. Its not Movie Maker but the file you use in MM. Most recording equipment use either FLV or MP4 files. You cannot edit FLV files in MM, but you can change them. I use Ultimate Video Converter to change my files into a more suitable format(MP4,MPEG4). After that go back to Movie Maker and you should be able to edit again, without any problems. Here the link to there website if you curious and true me its safe. http://uvideoconverter.sourceforge.net/
I would not have recommended it, if it wasn't safe. One more thing, use the HD option. If your computer can handle it.