The supported file formats by Windows Movie Maker are the following:
Video files: .asf, .avi, .wmv
Movie files: MPEG1, .mpeg, .mpg, .m1v, .mp2
Audio files: .wav, .snd, .au, .aif, .aifc, .aiff
Windows Media files: .asf, .wm, .wma, .wmv
Still images: .bmp, .jpg, .jpeg, .jpe, .jfif, .gif
MP3 format audio: .mp3
You will need to convert your original file to the one compatible on this list. You can try an advanced solution like Xilisoft Video Converter Ultimate to bring your original file to a compatible format mentioned above. Then you can import it into Windows Movie Maker.