Windows Movie Maker messes with quality of video?

I was editting a Minecraft video (How steriotypical, right?) and I was adding in 3 videos into Windows Movie Maker to trim them down into short clips for a montage but I noticed that 2 of the 3 videos had to "Prepare" and so I waited for them "Prepare" while I cut up the video that didn't need to. This video was HD as I recorded it, a sharp video but when the other videos prepared, they were about 240p or 360p for some reason. I watched the videos on Windows Media Player and they were fine, HD, but for some reason WMM just messed them up. Does anyone know how I could fix this?

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