Audacity won't open mp4 files

Windows 7
Audacity 2.1.1
exe installer

I try to open an mp4, a message comes that it is an Advanced Audio Coding file, and Audacity cannot open this type of file. I believe this is happening because I allowed an anti-malware program to clean my computer a bit too well. How do I reset Audacity?

Something called FFmpeg libraries has to be installed into Audacity before you can open mp4 files in it ,
[ you only get the MP4 audio in Audacity of course, not video ] …[b]How_do_I_download_and_install_the_FFmpeg[/b]_Import.2FExport_Library.3F