Windows Movie Maker doesn’t work in Windows 10

I've upgraded to Windows 10 a few days ago. WMM was working just fine in Windows 8. I use Fraps to record videos, and then I edit them using WMM before uploading to YouTube. WMM supported AVI files (AVI is the type of file Fraps uses). Since I've upgraded to Windows 10, WMM no longer supports those files. An error code comes up telling me it's not supported and the file may be corrupt. It says in order to fix the issue, I would have to remove it from WMM. I do that, and it happens all over again. I have tried to find programs that convert AVI files to MP4 files, but none of them work. I want WMM back; the way it was before this upgrade. If I would have known that WMM would not work in Windows 10, I definitely wouldn't have upgraded.