How To Transfer Realplayer Videos To Windows Movie Maker?

Ive Been Trying To Put One Of My Movies I Downloaded On Youtube Using Windows Movie Maker But It Always Says 'The file C:\Users\user\Videos\Osmosis Jones.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. HOW DO I RESOLVE