XP Movie Maker vs Vista Movie Maker

I realize Windows Movie Maker is not the greatest choice for rendering video, but for now it is all I have.

Here is my problem: Rendered a video into XP Movie Maker then tried to import it to Vista Movie Maker. Vista reports a codec is required to be downloaded before said video can be imported.

First, why would Vista need a required codec that XP has, apparently native? Both machines have all current updates.

Second, was referred to Codecs.com by HP to download a BETA version which would remove all apple components from machine. Not acceptable as I use iTunes for music and activation of iPhones online.

Does anyone have an answer that is not destructive?