While Windows Movie Maker says that it supports popular video formats, but the truth is the application doesn't natively support those video formats until the proper codecs installed. Without having the proper video codecs installed, you may have issues with importing, editing and exporting video [MOV, MP4, AVI, etc…].
As a suggestion for downloading codecs, http://www.free-codecs.com/ has an array of codec packs to choose from. I have used the K-Lite codec pack [standard] for years, and I've had very good success; it even gave me the ability to edit FLV files in Windows Movie Maker. I have never had a problem.
I put this together because there are apparently a lot of people scared to install software if they don't know what it is. So, below is a step-by-step installation with screenshots so people can see what's going to happen before they do any thing.