Movie maker no longer recognizes added mp4 file

Windows media player no longer played some (but not all) .mp4 file after I repair the Internet Explorer. I resolved it by downloading/installing a codec pack from CNET Download.com. It has worked great since, being able to play video with other formats (e.g., flv) which I was not able to play with Media Player before. However, my movie maker no longer recognizes .mp4 file. Music (mp4) inserted (added) to my video is not longer recognized by Movie Maker. I got an error message saying "Cannot save a movie, "This file might be corrupted or in a format that Movie Maker doesn't recognize" Error: 0xC945002D." I am happy with the recently installed codec pack, but Movie Maker is compromised. Please help. Thanks.