Codec for Windows Movie Maker?

I would like to edit video clips from my Nikon camera but when I try to drag or import the clips from their file into Windows Movie Maker I get this message:

"The file can't 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 can I fix this?

There are many codec packs all over the internet, many of them are even free
Search, download.com
download and install.

For video playback, get VLC player, it will play many types of video extensions.