windows movie maker importing files problem

iam using windows movie maker to import files into my collections for something i bittorented. the problem is the importing only works for some of the episodes which i downloaded all at once in a one package deal, they all play in real player but for some reason it says i dont have the right codec for certian ones and i dont know which one to download.
plz help its getting anoyying

You must download the codecs from sites . Here is one from me www.free-codecs.com