VLC won't play .avi or .mp4 files?

Assuming you have a Mac from the tag, you should download Perian. It’s a plugin which gives Quicktime the ability to play pretty much anything. The AVI format you’re trying to play is most likely DivX format, which is supported by Perian. I’m not sure why you’re having problems with the MP4 however.

@troubleinharlem I think you should uninstall it and do a reinstall from scratch.

@Vortico VLC can play anything without any special plugins cause it has all of them built in already. I

The problem is most likely the file. Try to download it again.

VLC sucks. But unfortunately often seems like the best solution.

Just uninstall and redownload. I have to do this like once a year with VLC, b/c sometimes it just decides not to be good anymore!

After reading these comments. I feel like some information is left out. First of all VLC plays just about anything. If your video is not opening with VLC then theres a problem with the file your trying to view. Make sure the file is not corrupted. If the problem persists then try downloading the k lite codec (http://www.free-codecs.com/download/k_lite_codec_pack.htm) Codec includes every media file code available. It should work. If not reinstall VLC from scratch but make sure you uninstall it correctly.