discussed here; http://www.howtogeek.com/forum/topic/mp4-video-editing
It seems a better idea is to convert the file to avi first, then use WMM to re-size the file.
However that was 4 years ago and I don't know if things have moved on since then. You might want to consider Virtualdub which is free, but is an advanced video editor.
I took some videos with my cell phone and need to decrease the file size, which I assume would mean shorten the video. I opened it in Windows Movie Maker, which at this point appears to be a cheap piece of garbage. Any clues how I can do this without having to buy Final Cut Pro or any of the high end programs?
For some reason when I open them in Windows Media Player, it's sideways. I got it to rotate in Movie Maker but that's about it.