I suggest you to download YouTube video with Replay Media Catcher or any other software, web-site,.... And then edit downloaded FLV video with freeware AviDemux or VirtualDub instead of MovieMaker.

Replay Media Catcher is FREE for downloading YouTube videos.

You can also save YouTube videos with freeware Freecorder 4, Jaksta Free Media Recorder.

And you can use web-sites like http://download-youtube-videos.org/ to download YouTube videos for FREE without downloading any software.

More about downloading from YouTube:
How to download from YouTube
Download YouTube videos without downloading and using any software
Download youtube videos with bookmarklets for free
Download secure RTMPE videos from YouTube

And more:
http://stream-recorder.com/forum/tags/youtube/