Problems with half screen going green when I use movie maker

Hi there
I've looked at a couple of forums dealing with this question but still haven't been able to get my windows movie maker to work properly.

It used to be able to accept mp4 video files from my phone perfectly well. Then for no apparent reason it started making half the screen green when I add them. I currently have a compilation saved in a windows movie maker file which perfectly plays the clips that I'd added up to a certain date but when I add new ones the green half screen problem happens.

I've tried downloading the MediaArea net program but have no idea how to use it to work out if codecs are missing. Also one of the help pages on this forum recommended something with a name like 'fix video crash problems' from this page -

How to use Microsoft easy fix solutions

Any one have any ideas why this problem is happening and what I need to do to fix it?

Thanks!