Windows Movie Maker Import File Error

I can’t use my movie maker every time I imported a video or sounds an error message always appears. Even when imported other video file format and it always hanged. What should I need to do?
"Windows Movie Maker
The file C:UserchararatDocuments12_51 by krissy and ericka FULL song and lyrics on the screen.flv cannot be imported because the codec required to play the file is not installed on your computer. If you have already tried to download and install the codec, close and restart Windows Movie Maker, and then try to import the file again.
OK"
